Actually all of the boxing exhibits for the 1920s have dated backs with bios and/or stats on them. That is why the sets are readily checklisted and the dates I listed for them are absolutes. Here's an example, a 1926 Loughran:
The fact that PSA does not grade them is an asinine reason not to acknowledge their existence. If the registry is the only consideration then don't call the list a RC list, call it a PSA registry list. I don't know why PSA won't grade them but SGC and BVG will.
As for PCs, they can often be dated precisely based on copyrights and postal usage. Like this Zercher, which was mailed several weeks after Willard took the title:
here's a PC of Dick Tiger that predates the card listed above by at least four years based on the titles listed [and not listed]:
Here's a Jimmy Bivins:
Strictly speaking, the prevalence of foreign cards on the list is problematic. As I recall it, a rookie card had to be available in the USA to be listed as other than an XRC.
1953 Ray Arcel promotional PC featuring Carmen Basilio
