The German aid to the Ottoman Empire is a huge topic, and I will definitely revisit that topic several more times. There are lots of variations of markings and combinations of markings, and I am not sure whether the time periods of the different markings are well enough researched, but here is, at least, one example. (I have many)
Now, the first Gewehr 88 came to the Ottoman Empire along with the German battle cruiser SMS Goeben who was transferred to the Ottoman navy 16th of August 1914 and went on to harass the Russians on the Black Sea as the Yavuz Sultan Selim. Her weapons, including Gew88, remained on board.
Then, sending much needed military aid to Turkey became difficult and dangerous. The only route was by ship, and with hostile naval forces around, the long sea voyage became, to say the least, perilous.
When Bulgaria joined the war in 1915 on the Central Powers side, though, the situation changed. Now aid could be sent over land by rail, safely, and tons upon tons of German equipment was sent to the Ottoman Empire. I believe this example is among the weapons sent then.
There is a plethora of different Ottoman markings on these, with a crude crescent on the lock (early?), with crude or far from crude crescent and star on the bolt handle, and a few others.
This one has no Ottoman Crescent or crescent and star markings. It only has the rear sight marked with Ottoman numbers (no, not Arabic) instead of our standard numbers. These are old Turkish numbers, that differ from the old Arabic as well as the Arabic numbers that we use in Europe and America.
These rear sights have been claimed to have been renumbered in Turkey, but I believe (have been told) that they were fitted with new rear sights in Germany before being sent to the Ottomans. Many retain the original German rear sights, but quite a number have these with Turkish numbers.
It is in the 88/05 configuration, as the Ottoman used Gew88 are, with fittings for Mauser clips added to the lock and a cutout in the lock to accommodate the Spitzer bullets. A snap-on cover is added to close the hole beneath the magazine, which was not needed after eliminating the Männlicher type clip that had to fall out through the magazine bottom. This cover is the 1914 dated type.
The rear sling mount has had a crude sling loop added, made of thick steel wire, to accommodate the Turkish slings, possibly added post WWI, but the sling on it is standard German Gew88/98, original.
These went on to be used after the fall of the Ottoman Empire. They were used in the Greco-Turkish war in the 20s and remained in use up to at least the 1950s. Many were “Mauserized” post 1928 (I have one), but plenty were not.