Hi Rick!
I'm afraid that I don't have much insight to offer, but I'll do my best. Those sure do look like JCM21 cards, but I've never heard of them with blank backs before. It's possible that they are JCM21s that didn't get printed on the back. Quality control for Japanese baseball cards circa 1950 can't have been too good. There are lots of uncatalogued sets, but it seems weird to me that they would issue an identical set that's just missing the printing on the back.
As for the thickness, again I can offer only a guess. Menko cards from the early 50s tend to be relatively sturdy, a few of them are almost Goudey-like. Which makes sense, since they were intended to be thrown at the ground. They weren't baseball cards exactly, they were toys that had pictures of baseball players printed on them. Despite how it's catalogued, it's not clear to me that JCM21 is a menko set. They've got no menko numbers, and no rock-paper-scissors symbols. It seems to me that JCM21 is really just a deck of playing cards. Now, I don't have any cards from the set, so I can't say anything about it's thickness. But if JCM21s were meant to be used as playing cards and not menko cards, it's no surprise that they would be much thinner than is normal.
Sorry I can't be any more help than that. Hopefully some of our more knowledgeable collectors can chime in.
You know, I might as well include a card in this post. Here's an upgrade to my Futoshi Nakanishi. Or, well, 'upgrade' is the wrong word. On my
other card he's sharing the spotlight with Takahiko Bessho, whereas here he has the whole card to himself. I'm not sure which set this card is from. It looks like it belongs to several of the JCM12 variations, but the only one that both has a border and pairs Nakanishi with this menko number is 12e, which Engel describes as "painted looking". This card doesn't look especially "painted" to me, but that's my best guess. In any case, it belongs to the JCM12 family.