The factory mod is off and sure enough the barrel has been crimped by the parting off tool at the muzzle.
The barrel diameter is 16.2(ish)mm. The reduced shank is 13mm.
The factory thread is m12x1.25.
I've ordered a brass adapter listed as 13mm. If that works out I will cut the muzzle back several mm and crown it by hand, test it and if improved source a mod and scope.