Where did that number come from? I'm not an authority on this, but 50 might be a little high. Remember they're only original once, and you don't want to blow 60 year old speakers.
I read it in the pdf you found, it said "....35 watts may cause distortion and if prolonged possibly damage. However amplifiers having a greater power rating----say 50 watts---will be perfectly satisfactory but should not be operated at their full power capability." Yeah I'll be careful with them. I read that on 2 or 3 pages in that pdf. I think they are good for a larger room.