I've been trying to make some trade offers for that '56 Bill Sarni, as well as some other '56 play at the plate cards.
I've noticed they are unlocking a lot of '60s cards with a pretty fair amount of '50s cards mixed in as well. Earlier I saw 6 of the 10 cards on the "recently unlocked page" were from the '50s. This is a screen shot from just a minute ago.
Two from the '50s, four from the '60s and some dude from 2010 named Strasburg.
I'd give it a shot if I had any codes.