A reader -- whose name and contact information I am embarrassed to admit I lost in my jumble of email accounts -- has inquired about a possible variation on the regular-issue (#50) 1964 Topps Mickey Mantle card.
On his card, authenticated by PSA (though not as a variation), there is no period after the "Y" in the team name at the upper-right on back. Typically, of course, a period appears there.
To try to get a feel for this anomaly, I contacted a major dealer in vintage cards, who has since asked that remove his name and findings, which were pretty much in line with my personal opinion of the status of the card in question. I believe the period disappeared as a result of over-inking of the red-orange background and/or deterioration of the printing plate. It is likely that many cards exist with not only this period, but also other punctuation is differing stages of being all there or not there at all. This result is typical of reverse (white on color) printing in that era.
For the present, I'm not going to add this to the checklist for 1964 Topps in the Standard Catalog of Baseball Cards, but I'm certainly willing to listen to pro-inclusion arguments, and will revisit the subject if it ever appears like the "no period" card is finding acceptance in the marketplace.