Interesting discussion. I have little to add but maybe it will be of value to just recount my knowledge of RJT smiths and stamps and showato etc. I am not the guru, but have had 40 odd years of interest in RJT and even abandoned koto-shinto-shinshinto for them alone.
1. About RJT rules. I suggest everyone read the translation of the rules and regulations that RJT smiths had to obey (Brian has it listed in the "interesting PDF's" section at the top of the index). In this you will see how strict the RJT scheme was on the smiths. You will see that everything was covered with a rule and that ANY left-over, rejected or broken/condemned tamahagane MUST be returned to the authorities. It does not specify anything about showato or using "some" tamahagane for other production. Because there is no mention of any showato process it is hard to imagine where the theory comes from.
2. About showato, I think it safe to say some competent gendai tosho made them...but I would say pretty definitely not as part of the RJT scheme. I personally have not seen a showato with a known RJT smith's mei on it.
The only written evidence I have of this happening is from a list of 44 tosho who were members of the Northern Japan Swordforging Union. This was run by Kasama Shigetsugu and member smiths (including RJT smiths) in the northern prefectures above Tokyo sent their swords to his warehouse in Tokyo and he mounted and marketed them to military officers...I can't say if they put any in-house stamp on them during this preparation process. A note in the margin of this list states that 2 or 3 members also made showato. From my own research in Japanese books I would say that these 2 or 3 "dual" tosho were BABA Tsugukiyo (RJT), YAMAMURA Sukeyuki (RJT?), and YANAGAWA Shoshin (Ushoshin) (RJT?) (Special Ranks - Betseki), but I would be surprised if any used any portion of the government supplied tamahagane or pine charcoal allotments to do this as the rules are so strict. For any given portion of tamahagane/charcoal supplied, the smith had to produce a required number of swords and return any left over material....another point is that if they did use RJT material would the sword be gendaito or showato? .
3. About stamps. This list includes the 2 Yamagami brothers, Akihisa and Munetoshi, of Niigata. Both were RJT and both also made private order gendaito. They (a) used tang "production?" numbers from 1941; the (b) star stamp with numbers from c.1942 and © also the "matsu in a circle" stamp with numbers from 1942 and (d) the combination of matsu/numbers/star is also found together.
It is my opinion that the matsu stamp and number system was strictly that of the Yamagami brothers and as they worked together, these numbers "overlap" each other's tangs. I think the number system changed by 1944 when the more common RJT system of a katakana prefixed number appears. I think this may have been imposed by the RJT authorities and this style appears on many RJT produced tangs. I must say I can't remember seeing this numbering on any Seki Nakata Kanehide RJT tang however, so maybe not all RJT smiths had to use them. Kanehide does have katakana prefixed and also alphabet prefixed numbers (prob. assy. numbers) on his tangs but always painted. Whether bunches of numbers were allotted to only certain RJT smiths or they were a local "code" system I don't know.
4. About the "other than star stamps" seen on RJT tangs, I think it is, as mentioned by members above, simply an inspection/acceptance mark, but seems limited to the major arsenal sites. Other than the Yamagami brothers' matsu stamp, other "rural" RJT smiths seem to have nothing more than a star and tang number and sometimes not even the number...I suppose this shows they were mounted/distributed through facilities other than the major Army facilities, probably private distributors. I have seen Seki Nakata Kanehide blades with star stamp only, star stamp with 1 or 2 tiny "Seki" stamps and also one tang with tiny "na" stamps (sometimes on the mune and also on the date side). I think this simply denotes which "arsenal facility" the blades were dealt with / mounted through. It has also been seen that other RJT smiths have been seen with stamps such as "ko" (Muto Hidehiro - Kokura),
So, although we never say never with nihonto, I think the stamps are perfectly legitimate and should not disturb collectors. The idea that they denote "part showato" is IMO incorrect and the idea that "part-showato" RJT blades are around is also 99.9999% (IMO) unlikely.
I will say however that I have seen a very good quality looking choji hamon blade by Kanehide in his usual metal scabbard Type Contingency Standard mounts that while correctly signed and dated by him had no star stamp or tiny stamps on the tang. A careful inspection showed that this magnificent sword has a very fine ha-giri in the choji. It is this that explains the lack of acceptance of this blade to the RJT scheme and thus...no stamps. The blade was permitted however to be sold into the military. This blade alone kind of confirms how strict the RJT inspection standards were and so it is unlikely that "part-showato" swords would get through the system and get a star stamp or any other stamp.
Hope this helps,