I have not found any formal documentation, yet, demonstrating the sanction that Wembley Pro was given, from the ILTF. I do have this statement by Bill Tilden indicating that the World Pro Championships in Berlin (1932-33) was officially sanctioned.
AP report of January 18, 1934:
“Nusslein is the holder of the professional world’s title as recognized by the International Federation of which the United States Professional Association is not a member,” Tilden said. “Vines and I are playing for the indoor world’s professional championship, which is held by me, not Mr. Nusslein.”
It's just unclear, by "International Federation", does Tilden mean the ILTF, or an international pro body?
The US Pro governing body mentioned by Tilden also comes up here in this AP story on Jan. 22, 1934:
FOREIGN TENNIS ACES COMPLAIN
European Racquet Experts Object to Tour by Tilden, Vines
NEW YORK, Jan. 22—(AP)—The current Bill Tilden-Ellsworth Vines tour has no sanction from the National Professional Tennis association, J. P. Allen, secretary of the organization, explained today as a result of conflicting developments.
Several protests were received from foreign professionals, including Hans Nusslein of Germany, when the Tilden-Vines tour went on the road, billed as being for the “world professional championship.” Nothing was done about this, Allen said, for the simple reason that Tilden’s troupe is a private enterprise and in no way connected with the national association.
Big Bill Indoor Champ.
“Tilden holds the national indoor professional title, won at the tournament this winter in Philadelphia,” said Allen. “Nusslein is the world outdoor pro title-holder, having won the championship tournament in Europe last summer. Neither of these titles can be won or lost except in the annual tournaments. They are not subject to ‘challenge matches.’ Tilden Tours, Inc., which has most of the star professionals under contract this year, is conducting and billing its attractions to suit its own purposes.”
March 14, 1934 in the Palm Beach Post:
Hans Nusslein, world professional champion who gained that honor by defeating Bill Tilden at Berlin last fall in the International Tennis federations recognized tournament, made his appearance during the afternoon and stroked a clear-cut victory over Jimmy Kenny, of Palm Beach and Newport, 6-2, 6-0, and 6-2.
January 22, 1934 in the Palm Beach Post:
NUSSLEIN WILL ACCEPT TILDEN'S OFFER TO PLAY
The world's outdoor tennis professional is very willing, in fact anxious, to play Bill Tilden a series of matches to test the supremacy of the two.
But, it is doubtful that any title will be at stake.
Hans Nusslein of Germany, wintering at Palm Beach and located at the Everglades club clay courts, is the world outdoor pro tennis champ. He won the right to call himself that at Berlin last September in the finals of the International Professional Tennis federations tournament recognized for the world's title. Tilden and Bruce Barnes won the doubles title there.
Tilden is holder of the national indoor title, an honor he gained at Philadelphia several weeks ago. But, as far as being world's indoor champion--well, that honor is questioned by Nusslein who so far has failed to find where the International pro federation has recognized such a tournament.
Right now it's Nusslein and "Vinnie" Richards who hold the major professional titles so far as singles are concerned. Nusslein's title is impeachable, that explained above. Richards won the American title last fall, defeating Frank Hunter in the finals.
Nusslein, questioned at the Everglades club, said he is more than willing to play Tilden, in answer to the latter's statement made at Cleveland the other evening. "Tilden is the greatest player the world has produced," Nusslein said, "and I firmly believe him to be at the peak of his game now. He is better than when he played in the amateur ranks and I do not underestimate his ability as the foremost player, professional or amateur.
"However, I believe I can defeat him in a series of matches and wish to remind Mr. Tilden that I did just that in Europe last summer, winning seven of 12 matches, including the championship at Berlin. It is true that he defeated me more often than I he during the American tour in 1932."
Nusslein could not understand Tilden's reference to a sore arm he had at the Berlin championship matches, saying that he and Tilden played together only several hours prior to championship matches and that Big Bill was at the top of his game then and very apparently at the peak during the tournament.
"Tilden, about eight days after the championship, was defeating Cochet and Plaa in straight sets at Paris," Nusslein added.
If, and when they play, and a title is at stake, the tournament must gain the sanction of some ruling association, either the American Professional association or the International Federation.
And, it looks like any impending match will not be played before April 1, that because of the Tilden's tours –T.P.
Nusslein was often referred to in early 1934 as world pro champion. A number of reports in '35 state that he lost that title to Vines at the inaugural Wembley championships of 1934.
United Press report on that '34 Wembley:
Ellsworth Vines Defeats Tilden For Tennis Title
LONDON, ENGLAND (U.P.)—Ellsworth Vines, professional tennis champion of the United States, Friday defeated Big Bill Tilden 9-7, 7-5, 6-2 in the round robin final at Wembley and won the world professional championship.
In a battle regarded as one of the greatest ever seen in London lawn tennis, Vines’ youth was the deciding factor. Tilden began to tire in the second set and lost his masterful stroking. He netted balls frequently and was confronted at every turn by Vines’ counters on drop shots and hops, which Tilden tried to use to slow up the game.
The Clarion Ledger (Jackson, MS) on March 10, 1935:
Hans Nusslein, former German professional tennis title holder and pro champion of the world until last year, joined Big Bill Tilden’s tennis troupe that will stop here for exhibition matches March 27…
Since Hans Nusslein first came into prominence in the tennis world by winning the German title in 1931, his rise has been phenomenal. He has held the world’s professional title, relinquishing it last year to Ellsworth Vines at Wembly [sic], England.
He had previously defeated all comers including Vines and Kozeluh to win the United States professional championship in August 1934.
Raleigh Register (Beckley, West Virginia) on March 24, 1935:
Every man in the troupe holds one or more world’s championships. Tilden is recognized as the greatest tennis player of all times; Vines is the present world’s professional champion; Nusslein was world’s champion until last year, when Vines wrested the crown from him at Wimbledon [sic]…
But Wembley possibly did not (yet?) have status as the world pro championship with the same certainty that Berlin Pro had. Nusslein still held the world pro title even after his defeat by Vines at the '34 Wembley, per this report in the Burlington Daily Times on April 4, 1935:
Although not generally credited with the honor, Hans Nusslein who is to appear here Thursday night at 8 o'clock with Big Bill Tilden's tennis professionals is the holder of the world's professional singles championship and the co-holder of the world's pro doubles.
Nusslein, a German, won the championship in 1933 in Berlin. Since no tournament was held last year, he still retains the title. An unusual fact in his winning the universal honor is that in the field of 164 entries he won in straight sets.
Now, Berlin Pro was actually held in 1934 as always, so when this article says that "no tournament was held last year," it means that no tournament in '34 was held to be the World Pro.
This was in American Lawn Tennis, a report from their English correspondent, about the 1935 Wembley:
Professional Tournament in London
It does not seem possible to say by whose authority the lawn tennis contests held at the Wembley Stadium in the vicinity of London during the week September 30-October 5 were styled “The World’s Men’s Professional Indoor Championship.” It will perhaps suffice if I say that they are not recognized as such by the governing body of the game in the country in which they were held and probably owe their title to the fertile brain of one W T Tilden, who was, in a manner of speaking, the author of their being.
So when did Wembley gain formal sanction, and what form did that take?