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warshawlawit is easy to pin down the issue date of the base exhibit sets, but the PC back set was issued in several iterations over the years from 1925-1929. The first wave has blank backs or backs that read "this side for correspondence". Next up are the same cards with backs that read "not to be used in exhibit machines". From there, at least two additional subsets were issued. These were referred to in olden times as set 81, set 82 and set 83. It is clear to me that they were part of a continuum of one issue from the same maker and probably are better understood as series, not distinct sets, as the back markings varied. I base my dating on the analogous boxing issue, which can be pinned down as to dates based on fighters' title changes and the existence of 1924 stats backs on the first wave of the cards. The baseball issue appears to have run for the same time frame and with a similar pattern of changes.
I bring this up here because there are very early cards of Grove and Foxx in the issue, possibly some of the other 1930s players (I don't collect rookies so I don't know), and we have no way of knowing precisely when they were issued in the timespan. For example, the assumption is that the A's greats were issued ca. 1929 when Mack had that insane team, and I know that the Lefty O'Doul was issued after he was traded away from the Giants after the 1928 season, but more precise than that we cannot be. I wish I had a few postally used cards or some advertising stock from the sets but I don't.