There is little to say about this particular rifle, because the OP shows so little useful information, we do not even know the range of the rifle... but what is known is that many crescent marked MO/1918's exist that show signs of interwar German service. Obviously they never left Germany, if for no other reason than after 1920 Germany never imported rifles, certainly not x-German made, and it was against German law to import or export military equipment of any kind after 1927 (a law passed to appease the English, French and Belgians who had just removed the IAMCC). Prior to 1927 the Germans are known to have exported much military equipment, all illegally, some caught, some not, mostly through the Dutch, but also covertly from German ports. No "imports" have ever been recorded so far as I know.
So, it seems the crescent was applied in Germany, not always, the purpose is not known, but many Turked G98's do not have them, but most do... I see no rhyme or reason for the application in surviving examples.
I suggest that what probably occurred is that between the time the rail line was opened up to Bulgaria-Turkey (when Rumania collapsed in early in 1917) the Germans pushed through as much munitions as possible (Bulgaria & especially Turkey were in dire need of munitions, one of the main goals of the campaign was to secure a reliable supply line to them) and when it became clear the Bulgarians were about to collapse (September 1918) these rifles were enroute and sent back, - some evidence of this exists in the form of the vast majority of MO/17 from the d block through MO/18 g block are Turkish used rifles and after the MO/18 g block they are increasingly German or Polish. (though within this Turk range "German" rifles exist in small numbers)
It is well known that Germany had no rifle (G98) shortage by wars end, many were not used, languishing in depots, these returned Turk rifles probably sat in their original boxes and considering the numbers of post g-block rifles that exist, probably were largely destroyed between 1920-1924 (MO/18 goes to the t block, less than a dozen are known in all the blocks between h-t)