I can't tell you the year of manufacture, but I have a Diapason 6'1" grand from 1983. I've had it since it was new, and it's a really lovely piano, comparable in my view to Kawai or Yamaha. Diapasons were made by Kawai in Japan as a second label, slightly less expensive, and sold in countries such as Australia (where I am). It has a particularly clear, strong and resonant bass. I've never seen or played a Diapason upright. Diapasons don't seem to have much resale value as most people have never heard of them. Good luck Marcin, let us know how you go.
I think MarcinBiela is a piano dealer in London, England/UK, who is looking for info on the age of the Diapason 125 upright piano, which he already owns, apparently. That is the way I read his thread.
Anyway, you are right about the resale value of obscure and little known piano brands. Even if the brand is a very nice, good quality, piano, the lack of brand recognition does indeed affect resale, to an extent.
Also, as MarcinBiela has discovered, finding information of some of these obscure brands can be difficult. I once owned a Tokai G180 grand, that was actually a pretty decent piano, but still an obscure and little known brand, also made in Hamamatsu Japan. I think I actually helped to put Tokai pianos on the Google map by asking so many questions about it here on PW. Google "Tokai piano" and see what comes up...
Also, @ MarcinBiela, welcome to Piano World!