Andres it's really hard to find them. I am usually a Omega moderator on WUS, just visiting you here, and we get a lot of questions on the SM300.

And as a visitor, it is of course rude to write in English, but my German is so limited in writing. But feel free to reply in German. I can read it very well

Mine is made of NOS (new old stock) parts from a company in Australia named Watchco. They have made a few, and they do sometimes have one for sale. I was lucky to get mine second hand in England this year

Watchco

Jon