Speaking about the W501 Zeb Terry (which I don't own) that I showed previously, here is a run I was fortunate enough over the years to assemble of his cards in the E121/W501/W575 family. I think it illustrates the differences between the various sets, and helps amplify the spacing remarks Val previously made. Even if you are not good at identifying if a card is grainy or not grainy, or its thickness, both of which I sometimes have difficulty with, or if all you have to judge a card are scans, it seems like the below noted similarities/differences can help us spot what any given card belongs to which set.
e121terryquad 001 (640x275).jpgw501w575trio 001 (640x272).jpg
e121terryquad 002 (640x274).jpgw501w575triobacks 001 (640x267).jpg
The first 3 from the left on the above scans are all versions of the E121 Series of 80 set. Note the last line on the backs are all in different style fonts. And the third one over has its first line ending in 'eighty', instead of 'the'. Now note that all three of these separate issues of the Series of 80 have the close spacing between the player name line of text and position/team line of text. Also note that Zeb's position is spelled out as '2nd B.'.
The fourth card from the left is Terry's E121 from the Series of 120 set. The first line on its back includes '120', which is a basic way to identify that it is indeed from the Series of 120 issue. On its front note the wider spacing between the player's name line and the position/team line. Also note that his position is identified as 2B, which is how American Caramel handled the position abbreviations in this set.
So much for the E121 background info in regards to the W501-1, W501-2 and W575-1 group of cards. As can be seen in the back scans, the final three cards in the row all have blank backs. The fifth card in the row is a W501-1 strip card (traditionally identified as simply 'W501'). This can be easily identified (if not cut from the strip it came from too close to the photo at the top border) by the 'G-4-22' in the top left border, and its card number at top right. Note that is has the same spacing between lines on its bottom front, as Val previously pointed out, and the same position abbreviation as the E121 Series of 120 example to its left.
The sixth card from the left is basically identical (spacing and position abbreviation) to its W501-1 friend to the left of it, as well as these same specification as seen in the E21 Series of 120 example. The difference, as visible in the scan, lies only in that it does not include the 'G-4-22' and card number on the top border. This Terry example is from the newly recognized (by SGC) W501-2 set, that Rhett identified and designated in an article many years ago in the VCBC magazine (I believe the VCBC...I don't have a copy in front of me). These, like the W501-1, were probably issued in horizontal strips.
The final card, at the far right, is an example of the W575-1 set. Note that it has spacing and position abbreviations just like the three E121 Series of 80 examples. These W575-1 cards are likely to have been machine cut. The card just to the left of this W575-1 card would have traditionally been identified as W575-1 as well, but because of similarities to the W501-1 set (and its top border difference), it has earned its own W501-2 designation.
Let us know Rhett if this all sounds correct, and hopefully this can help folks identify all this jumble easier.
Brian