Be careful, John, your gonna get hooked on matchbook covers if you aren't careful!
John Spano wrote an excellent article on matchbooks for Gridiron Greats that discusses the various years and difficult ones to find. It's issue #39 if you want to check it out.
In summary, here's how to tell the difference between the years.
1933 - easy to tell, silver color. Bio info has either a green or pink background.
1934 - tan, red, green, or blue color. Picture is colorized. "The Diamond Match Co. N.Y.C." is printed near the striker
1935 - same as 1934 except the pictures are NOT colorized (they are B&W) and "Made in U.S.A. The Diamond Match Co. N.Y.C." is printed near the striker
1936 - vertical bars on R and L sides of player photo differentiates this set of the the ones before it.
1937 - exactly the same as the 1936 design. For players in both the 1936 and 1937 set, I believe the only way to differentiate them is that their bios are different. Compare this George Musso bio with the one above.
1938 - back to a silver background but with a yellow football in the center of the matchbook
I don't have any scans of this set but you can find them online pretty easily to see what they look like.
jeff