I don't underrate Ramillon or Plaa, they were good players, but surely not big ones, they never won anything big (Frenc Pro 1931-32 were weak titles, we don't even know if they really happened
). Plaa won in Berlin 1932, which was a pretty good tournament but not a Major in my opinion (Tilden was past his prime, Nusslein was not in his prime yet, and obviously a lot of strong amateurs were missing).
Talking about Nusslein, the 1933 World Pro can't be considered a Major in my opinion: another pretty good tournament, but Kozeluh and Tilden were 38 and 40 at the time (and Kozeluh was not a big champion in my opinion).
In 1934 he had an amazing victory at US Pro against Vines, but we know that Vines was not unbeatable on clay.
On other Pro Slams he won, he faced mainly old players, while Perry and Vines were on tours (moreover, in my opinion Cochet never shined as a Pro: he had some good moments, but surely nobody will remember him for his Pro career).
From my point of view, Nusslein won only a proper Major, Southport 1939, but I can accept who says he won two Majors, for his 1934 win over Vines, which was surely a big achievement. That said, I really can't credit him more than two Majors.
So if someone asks me about Nusslein, I will introduce him like this: "an underrated Pro player from the 30s, particularly strong on clay. He won several tournaments in his circuit, even if many of them had depleted fields (because at the time amateurs had another circuit, and the strongest Pros preferred touring). Anyway, with his not-so-rare victories over Tilden and Vines, he can clame a place between the most distinctive players of the 30s. His biggest victory was Southport 1939, an English tournament on clay, which was surely a Major in that season."
I can say he was a sort of Muster of the 30s: enormously strong on clay, but only one Major ;D