Thanks to you guys and all the info you gave me to further my research, I have come to learn a lot about this sword. Thank you very much I also noted that you two guys are exactly the people I was looking for - I had a look at your profiles, you guys are not only years ahead of me, you are decades ahead in experience. So a special thank you for your uncomplicated and extremely swift help in solving a longstanding mystery.
I have now learned that the sword is likely a gendaito gunto (am I getting the lingo right?), likely from Masayasus workshop, probably not made by the man himself but certainly meeting his standards. I have sadly also learned that the blade has been mistreated... apparently someone inexperienced must have tried to polish it, as there are a lot of microscratches on the blade and I can't make out the hamon easily at all. The nagako has also been "freed" from rust. Oh well, a pity, but that was long before I met this blade. Don't worry, I don't intend to do anything with it other than carefully use some chamomile oil on it.
I have also learned that it is not exactly very valuable in the monetary sense. But I wonder if it might actually be valuable to you guys and other nihonto enthusiasts like you to maybe learn a bit more about the sword, as it is another previously unknown example of Masayasus work? I am fairly certain it is not a fake (I know for a fact it was bought at a knife fare over 25 years ago... fake nihonto apparently were not common then, and who would fake a Masayasu sword 25 years ago?). Would more pictures of it be helpful to you guys? It comes fairly fully mounted, missing only some little bits (like a tassle) and the leather cover of the wood scabbard is in really bad condition, but the blade looks functionally quite alright, although it is not sharpened.
If more pictures would be helpful or appreciated, I am happy to post it in a new thread elsewhere, if this helps you guys in some way. I don't think I will ever share your level of enthusiasm for Japanese swords to be honest... but as I am a big fan of Damascus Steel (think Manfred Sachse... have a quick look if you never heard of him, also a master swordsmith of a very different kind), I at least share your enthusiasm for beautiful steel. So if more pics would be a suitable way to thank you for your assistance, I am happy to take some.
Thanks again guys
PS: I don't have any papers or anything to officially authenticate anything. I have the sword and reliable oral history from the point it was bought at the knife fare around 25 years ago, everything before that is a complete mystery.