Falcon - you say “I know that the Swiss issued .38 Revolvers to their citizen soldiers…” Where did you get this information? Do you have any literature on it. I have been studying small arms for 55 years and I have never heard this before. Doesn’t mean it can’t be so, of course. Still, I have a lot of Swiss military manuals on small arms, several books on the subject, and a lot of articles (albeit the articles favor the long gun side of things) and have never seen mention of this. Can you please authenticate this statement?
Thun did make .38 special ammunition, primarily for match shooting and, I suspect, primarily to provide various shooting clubs with that ammunition, rather than to any military use. I can’t speak for now - times change - but the Swiss Government used to heavily subsidize shooting in their country. The only loading I have seen is full wadcutter, copied somewhat from the Winchester version of the same loading. It had a “D T” headstamp as well, although I don’t remember the date. The book “Schweizerische Patronenboden-Bezeichnungen, 1867-1985” by Michael am Rhyn, documents this load, but no other. I once had a box of Winchester .38 Special 148 gr WC ammo with a Swiss overlabel. I used it to keep a box sleeve for Swedish Tracers squared with the box, as it fit inside perfectly, and I did not have the inner box for the Swedish rounds. I showed it to a .38 collector, and of course, gave it to him when he went wild for it.
Is your blank a nickeled case, or plain brass? It sounds like a blank made from a Swiss .38 Special case - the measurements fit in except for the rim, but that is so little over to be of no consequence. Erlmeier-Brandt shows the rim measurements for .38 Special to be 0.425" to 0.444" and you show yours at 0.445". A difference of 1/1000th of an inch isn’t much in the measrement of a revolver cartridge’s rim, even when it is at the maximum.
The headstamp seems right for a Swiss .38 Special as well, as the only headstamp recorded by am Rhyn is from Thun with case metal supplied by Dornach, the same as yours.
If I prove to be wrong for some reason, I think you’re only hope for ID unless some Swiss answers this is to contact Thun and see if they will provide information. In the meantime, a picture is worth a lot of words - can you post a photo of your cartridge?