I come across menko cards pretty often - if you know where to look you can usually find some. However, baseball menko don't show up as easily as those with non-sport facades. I recently picked up a very large lot of menko with just a handful of baseball cards inside. The price was right and now I can share those with you!
I've arranged them by back type as much as possible. Color variations on backs are grouped together, and I've made no effort to separate by subject on the front. Some of the cards are duplicated, and may be shown together or split among scans. Sorry, it's the best I can do.
This first set I'm calling Sign and Space Backs. Most of the backs are red, with brown, black, green, blue, and purple also showing up. I believe color variances (different reds, for example) are due to fading, poor printing quality, or other inconsistencies in the inking process.
All of the cards in this group have a playing card in the upper-left corner and a sign of some sort on the top-right. This sign is related to driving, usually (not always). The meaning of the sign is explained below that. The middle has a picture of some sort of flying machine, either for space or military purposes (missiles, helicopters) but seem to all be modern or futuristic instead of historic. Its name is written on the right side. A janken (rock-paper-scissors) symbol is in the lower right corner, and the rest of the bottom line is a five-digit menko number. The publisher, Maruten, printed their symbol in the box containing the large image.
As you'll see, the subjects on the front can include cartoons and live-action shows. Ultraman makes an appearance in several of the cards.
There aren't so many backs, so many cards share the same back. The menko number, therefore, can't be used to identify the card.
This is by the largest group of cards in the lot. I'm not sure if it had to do with availability, the subject matter, or commonness. I really like the old images, even if I don't know most of the shows. And some of the cards have writing or highlighter on the back, showing that these did belong to a kid at some point. The writing on one of the cards says みつこ にしむた Mitsuko Nishimuta (probably should be Nishimura にしむら - could have been bad writing). Mitsuko is a girl's name, so maybe some girl bought these back in the 1960s or 1970s from the local candy shop to play with her friends.
Here are all of the cards in this series that I have: (the left-most card on the front scan is the left-most card on the back scan)