As noted above, I haven't seen the stat for 1st serves on break points before 1981. But before the Open Era there were occasionally vague references like these, reported in the Sydney Morning Herald the day after the Hoad-Trabert classic of '53 (Davis Cup Challenge Round):
"Hoad served 11 aces to Trabertís six. Many of these powerful services came at game-saving points."
"During the match [Hoad] saved nine game points, mainly with aces or strong serves."
Hoad saved 9 of 12 break points in all, per the Herald -- saving all 7 break points that he faced in the first two sets, both of which he won.
The "aces or strong serves" that Hoad put in on break points could have been second serves, but obviously they were more likely to be first serves.
Impossible to know for sure unless the full match turns up somewhere.