For a while after the war you could get lenses coated. If yours is number 099919 then it look like the late 1920's (sort of 1927-ish).
It should have a "0" stampted on it by the little screw by the infinity lock release button. That would mean it was standardised for any Leica to use rather than the one it came with originally. And that suggests it went back to Leica for the coating and standardisation.
The lens and bodies were standardised from 1931 onwards but a lot of people like to play mix and match with Leicas and the war meant many were cannibalised and remade from damaged ones. Then there's auction sites and wicked dealers. You get some oddities turning up from time to time and the war is a great get out of a corner card to play...