To expand on the confusion over counterfeits versus stock differences, in the late 1960s the company began to use whiter, thinner stock. You see it primarily on the '1948' HOF set that is attributed as a 1974 set in the Standard Catalog, and on sepia and black and white boxing cards. Both of these Clay versions are legit; the green was first:
The irony is that the greenie is the most valuable yet the sepia and the black and white (not shown) are harder to find.
What you have to check in terms of counterfeits when it comes to white stock is re-screening. Exhibits are half tone prints (little dots) made by photographing a real photo through a screen. This:
becomes this:
If you have the card but not the original art and you want to make a half tone print of it, all you can do is take a photo of the card through the half tone screen and then print it again. The drop in print clarity and quality is like a photocopy of a photocopy. Once you know what it is, you know what it is (yeah, I know, under "redundant" it says "see redundant").
Oh, and I put '1948' in quotes because the date is entirely a creation of some early cataloguer's half-baked research. The 1950 catalog still offers the set for sale. Same is true of the '1948-52' football set; the 1955 catalog still offers them for sale.
The post-1928 baseball issues are also rife with short prints and variations big and small.