Thank you for the thoughtful replies. I’m posting additional photographs of the sword and koshirae in hopes that the clues will help in identifying the stamp.
I purchased the sword in a local antique shop who recently acquired a militaria collection (including a lovely baby Nambu). I felt it was fairly priced and like the fact it was complete and it was not subjected to amateur restoration by grinder, sandpaper or buffer. The koshirae is typical. It does have a corded sarute which appears original and plastic same. The seppa, tsuba and fuchi have matching serial number which also match the blade (more on that in a bit). The detractors were that the spring clip was broken, mekugi was missing and the blade did have a few nail catchers.
The nakago has a two character mei on one side and the opposite side has, vertically stamped, the unidentified stamp (shown in the OP) and the three-digit serial number matching the koshirae serial number. There are no other identifiable markings on the blade beside the mei, serial number and stamp. A family friend translated the two character mei to “Sadaharu”. Google search indicated two hits, one from this board and another from the Japanese Sword Index. The NMB post does show a mei what closely resembles mine and the discussion was over whether the blade could be gendaito (http://www.militaria.co.za/nmb/topic/12259-sadaharu-gendaito/). The opinion tilted toward showato but was inconclusive without better images. After closely examining my blade and comparing it to my Shinto katana (in shin gunto mount) (http://www.militaria.co.za/nmb/topic/18526-hoki-no-kami-fujiwara-hirotaka-in-gunto-mounts/), I have little doubt it’s showato. The Japanese Sword Index Show Mei database is an amazing resource! The mei on that database also closely resemble the one on my blade https://www.japaneseswordindex.com/oshigata/index.htm.
I have both Gregory and Fuller’s books on Japanese Military Swords and neither has a reference to the stamp on this blade. I’m hoping that an identification of the stamp can lead to further research on the sword. It would be nice to know where it was manufactured. Perhaps the smith worked at a particular arsenal?
I am a notice collector. Any corrections to what I tried to describe, thoughts/observations (supportive or contrary) and suggestions on what to look for and where to look is very much appreciated.