Oh boy...where do I start?
This is my favorite Exhibit set and I've been collecting/studying it for about 20 years.
The set is undoubtedly of the longer time frame (25-31) than commonly thought, unless the card makers were friggin psychics, since there are team changes reflecting trades and initial issue cards that did not reflect trades before the traditional starting date. The company also had a nasty habit with multi-year sets of pulling and replacing cards, so there are shortprints all over the place. For example, Speaker is in a Senators uni listed with the A's. That is a 1928 card and it is moderately difficult since he retired part way into the season. Since he has a silhouette card too (discussed below), it also seems likely that the silhouette variations were created in 1928 and issued for a part season before new cards replaced them for 1929 (I don't think it is coincidental that there are six second poses and eight silhouettes--I believe the new poses and 2 new players (Klein and O'Doul??) replaced the silhouettes).
Fronts
The Old Cardboard gallery has a complete set of fronts to view:
http://www.oldcardboard.com/w/exhibi...1927-29gal.asp
Note that their 4 on 1 gallery is incomplete. Here is a complete one:
http://imageevent.com/exhibitman/4on1pcbackset
There are two forms of front variation: new images and reworked art.
The first reworks are the pasted on labels on Cobb, Frisch, Hornsby, Peckinpaugh and Rouch. The company literally pasted a label on the card fronts and reshot the artwork. There are varying levels of difficulty on these. Peckinpaugh was on his last legs so his card is a short print. Cobb too was a short print (though not as bad) as his career was nearly over. Hornsby is most common one to find.
The other rework are the silhouette fronts. There, the company cut away the background and put in a solid background instead. Again, see the Old Cardboard page for contrasting examples. The cards Scott put up that SGC labled "plain background" are examples. You know they came later than their counterparts because the original photos had to be destroyed and then reshot to make them. They are short printed, I suspect because they were replaced with new poses of popular players and new cards of up and coming players (like Klein and O'Doul).
Both of the above appear to be ways to extend the shelf-life of earlier art, something the company is notorious for doing.
The other variations are big-name players with new cards: Gehrig, Heilmann, Hornsby, Simmons, Ruth, Waner.
A note on the 4 on 1s: they are reused art for the most part (a few exceptions--see if you can find them) and have the Gehrig and Waner portraits and O'Doul with Brooklyn (that's one of the non-set variations). Tells me that the 4 n 1s were issued around 1931 (O'Doul was traded to Brooklyn after the 1930 season) and that the portaits of Waner and Gehrig came after the batting poses. Now, the earlier 4 on 1s with the sloppy lettering and mixed sports used the batting poses, which is consistent with them being earlier issued. I suspect 1926-1928 since there is a known example with the printed coupon back.
Backs
The backs are the key to decoding the set printing sequences. The first printings were blank backed. Next came the PC backs with "This Side For Correspondence" printed on them. After that came the "Not To Be Used In Exhibit Machines" backs, later with "Made In The USA" added at the tops. For example, I have never seen a Hornsby Chicago with a blank or correspondence back and do not expect to do so since the trade that landed him in CHI took place after the 1928 season. Similarly, I would be surprised to see a Gehrig or Ruth portrait or a silhouette card with a blank back. The Gehrig batting may be a rookie card (if it had a blank back) but the portrait is not--it was likely a 1929 and later issue.
On rare occasions you will also find an ESCO corner coupon. Since the coupon stuff started in 1927 (hand stamped coupons are known on 1927 baseball cards and 1928 PCL cards, with printed coupons regularly found in later sets), you can peg some printings to no earlier than 1928 via the coupons, if you happen to run across one. Of course if you do you have a rare printing anyway.
Thats all I have time for now...gotta go to work.