I made an interesting discovery: Cards from the 50's rarely if ever gave you a look at the number on the back.
Here are all the cards I could find that showed the uniform number of the featured player:
That's it, just two cards. The 1956 card shows Dee Fondy's #40 and in 1959 we get a look at Lee Walls' #2.
So if you're looking for uniform numbers, skip the 50's.