So one of the most fun parts about collecting menko cards is that you often come across items that are not catalogued. My new research project is trying to figure out info about this new series. I at first came across one card, featuring Nagashima & Mori, and a space-themed back. Now I have found a 2nd card from the set, also featuring Nagashima (and his children?).
So it looks like:
a) This is a set similar to JCM67: 1958 Maru Military & Space Backs. Thematically, these two backs apparently also feature military & space themes (an alien from Jupiter and sidewinder missiles), but it is quite distinct in design from other "Military & Space" sets.
b) This might be a set of only Nagashima cards. Two cards is not a large enough sample size, of course, but there were a number of sets that featured only Nagashima.
c) Perhaps this is from 1958, making these all new uncatalogued rookie cards. The design of the front (and photo choices), remind me of other 1958 releases, and the "only Nagashima" sets were primarily from his rookie year.
I still have to track down the original photo of Nagashima and the kids. If this photo was used on post-1958 cards, it would disprove my date theory. And I want to see the photo to try to figure out what is going on with those kids' legs - it's a weirdly cropped photo! And I'll try to see if I can find other cards from this set, to see if they feature other baseball players.