I can't say I know when the bluing began with WMO, one would need an indisputable "Imperial" rifle featuring the trait. The last pure Imperial German WMO/18's are in the f-block, 3 of them, beyond the f-block they are either Republican, Turk or problem rifles... WMO/18 production is confirmed through the q-block, with a report in the r-block but WMO/18's after the g-block are elusive (or at least I haven't seen many - up to the f-block each block has one to two dozen reports, after that it diminishes to two or three per block - I think most WMO/18's that avoided Turkey were destoyed by the German authorities working with the IAMCC, - or more likely before they arrived as Germany declared huge numbers destroyed before they arrived and the IAMCC accepted most of the declarations, probably based upon records or facts, - not too many Germans at the time were eager to be militarists, especially industrial workers... )
I also agree (and wrote an article that should be on Gewehr98.com) that the crescents were applied in Germany, primarily because several have been discovered with Republican era work. These probably enroute and returned due to disintegration of the Balkan front (September Bulgaria collapsed); anyway, some crescent WMO/18's are known reworked by Republican Germany.
Regarding this rifle, possibly someone removed the blue on the receiver, generally unblued receivers do not have blackened letters. Four WMO/18 a-blocks which are also Turks have blued receivers and are like this one pretty much total matchers (at least so far as shown - including stocks which all are sanded but match)
Small matter either way, pretty nice for a rifle that went to Turkey. BC was helpful, especially the acceptance (C/K) which seems consistent late 1917-1918.
