the gas cap is clearly "continental style", meaning not very likely of British origin. So this is in accordance with the metric threads on the bike.
I think you have to reconsider the tank decal. An inscription like you suggested wouldn't make much sense on a German or French machine.
I had a look at the major manufacturers that used pressed steel girder forks such as DKW, Zündapp, Puch or Jawa but their designs are different.
So no new ideas from me, sorry...
Cheers
Andy