Gentlemen (mostly, anyway);
I hope all experienced a Merry Christmas, if not coming away from it all overstuffed and/or frostbitten.
FWIW, the dimensions on this one aren't something that I've looked into, and probably will not. However, whenever one discusses shotgun ammo it is good to remember that most shotgun ammo today (since early '60's anyway) is "saboted" with a plastic sleeve. The only non-saboted projectiles one might encounter would likely be rifled slugs. Much of this has to do with avoiding the leading of barrels and the non-deformation of shot (ergo keeping the patterns "tight"). "Shot" in this case is used to describe multiple round projectiles fired in the same cartridge.
The patina here on "20" does not strike me as ACW vintage although there could be several reasons for that.
And, although my expertise with ACW bullets is almost as extensive as my experience as a multi-mission moon-walking NASA astronaut, I'd venture to say MOST ACW numerical & letter marks I've noted in my very shallow exposure to same are cast as "raised" figures, not "indented" or whatever the proper term would be.
It would seem to me that this might be a case of "site pollution".
