As discussed, besides postally used examples, there are a few distinctions that can be made to help with the dating of these issues. The 1950s PCs are slightly larger in size than the later issues. Pre-1963 PCs do not include the zip code on the studio address line. Other than that, uniform style might be the next best option.
Here are a few of mine. Origination dates are due to other existing postally used examples:
Error ("Ronald") - 1969:
Corrected ("Rollie") - 1969:
Reggie - 1969:
Catfish - 1965:
LaRussa - 1964:
Interesting that all of my examples happen to be A's players (although LaRussa and Hunter are w/ KC A's and the others are from Oakland A's). It's probably coincidence, but interesting nonetheless!!
On my Want List is a pre-1963 example of Willie Stargell. I know at least one exists, since I was the under-bidder in mid-2013 on eBay. If anyone has one available for purchase/trade, please let me know.