Anyone specifically collecting T206s with Sat Eve Post stamps?

[swarmee]

I just found one (Engel, NY Amer) and figured I'd check to see if there is a master collector of these Saturday Evening Post stamped cards like there seems to be with Howe McCormack and other notorious stampers of early cards. I saw some long-defunct threads that discussed some of the cards owned by Saturday Evening Post stamps, and it looks like there were a few different "Agents" that had stamped collections.

[swarmee]

I'm not selling it at this point. Would rather work out a trade if these stamps are being collected by someone, or might start collecting this back stamp variation myself. Trying to learn more about it.
https://comimg.azurewebsites.net/i/Baseball/1909-11/T206/NA/Clyde-Engle.jpg?id=713923a3-3398-4acb-a8f6-c7f194aafc0f&size=zoom&side=back