I do not know of any other Japanese ammunition factories that made the 7 mm Nambu cartridge in Japan. Information on the subject of Japanese ammo factories is sketchy at best. Much material was destroyed during WWII, perhaps more so than in Germany, because of the structures of buildings and the huge amount of fire bombing that was done. Secondly, the Japanese concept of “a factory” was somewhat different than many of us think of the subject. Many of their factories were spread all over an area - almost “cottage industry” - rather than contained in one building or a single complex of buildings. I would think, though, that by the nature of ammunition manufacturing, that the Japanese ammo factories were perhaps a bit more confined to single buildings or building complexes. Third, of course, is that production of the Baby Nambu pistol itself was fairly low, and of course, that would reduce the need for cartridges and multiple cartridge-producing facilities for this caliber.
As to rarity, I would call the cartridges “scarce” now, rather than rare. Occasionally in not too many years passed, even some full boxes have shown up, bringing very big prices (500.00 up) and often ending up with a gun collector to go with a pistol, rather than with a cartridge collector.
I have a complete Baby Nambu outfit captured in the Philippines by a friend of mine during WWII. He had fired the rounds left in the two magazines, but all 14 of the cartridge in the loops of the holster. were still there. I have them with the pistol, although due to verdigres, I have long since removed them from the shell loops. I did want to obrtain another 14 for the magazines, once I became financially able to buy that many five buck cartridges, about what they brought at the time.
I then had no particular problem finding that many, out side of my actual cartridge collection, within the space of about a year. Obviously, it was a casual effort as well, not exactly an emergency need to find them.
There are variations in the cartridges, as would be expected for a round probably made for about 40 years. Rim thicknesses, shape and size of the extractor groove and bevel, and so on. Also, they are known with and without the stab bullet crimps at the neck. To my knowledge, all of the ball rounds made in Japan have a CN bullet. I forget off-hand what jacket my factory dummy round has.
Boxes are now hoard to find. I was able to find a box with a few rounds in it, perfect for the way I prefer boxes (empty save for one round to show original content of the box, since nothing is learned from the other 49 rounds in most cartridge boxes). I used the couple extra ones in the box for trade years ago, when I acquired it.
Rounds were made in Thailand, many with GM bullets and even nickel primer cups. There is an all-blackened round also, that may be a dummy. I seen a few in collections, but I have never pulled the bullet on mine to see if it has powder or not. Total cartridge weight is inconclusive. They are simply too rare for the rather “dicey” act of pulling bullets out of old pistol rounds (split necks sometimes the result of pulling, or putting it back in) and I would not risk that under any circumstances. Some of the Thai rounds have the look of having been made with totally refurbished .30 Carbine cartridge casings. Of course, Caveat Emptor on those, as it is the case used by those few reloaders of this caliber to form the original caliber. I have a couple of samples of those, one from a good friend who used to shoot his Baby Nambu with some regularity. I wish I had obtained his dies and components left when he passed.
Well, I wish I had some good solid information for you, but that is about all I know about the cartridge. I guess I should add that I have an unfinished aluminum case that could only be intended to have been made into 7 mm Nambu, but I would discount it as anything more than an attempt by a handy reloader to make some cases from scratch. I also have a red wood bullet blank, but unlike the 8 mm nambu version, I found these in a dime box at a gun show and they are, to me anyone, obvious fakes. I kept them only because it interested me that anyone would go to all that trouble to make them.
Hope this has been of some assistance, anyway.