Thanks for the tip MJL! I just checked the availability of this particular model, and there are only three webshops that carries it. All three are american, and two of them will not ship internationally. The one that will ship has the highest price by far.
GEPE is originally a Swedish company, but their distributor in Norway do not carry the cable releases.
In the mean time I have been using a release from Silvestri that I purchased from Linhof Studio not so long ago, and so far it seems to be working. It has a less steep cone than the "Leica nipple".
As I was writing this I suddenly had a bright idea (doesn't happen a lot, don't worry). Next time I will put a drop of Loc-tite (or equivalent) on the threads before I screw it in. Hopefully that will hold it in place, at least for a longer period of time.
As for the compatibility with Mamiya Press it seems that they have tried to change everything on the 600SE. Lens bayonet and film back locks are different, so why not the cable release as well!