I can't comment on whether a 88 Turk rifle is rare as I believe that is the most common Gewehr 88 you will find out there. The one variant I can imagine being rare is the Tufek 1935 modified Gewehr 88s.
What I have seen is that Gewehr 88s that were given to the Turks during WW1 were not re-worked to have their recievers blued and will retain their German originality (receiver in the white, German acceptance marks, receiver proofs and manufacture with date still present). You will normally see the rear sight re-stamped with Arabic numerals and will generally encounter a Turkish marked bolt with Z's in a circle. These are Czech replacement bolts that the Turks ordered in the 1920s to replace the ones that the British took out and never put back.
Gewehr 88s that received an arsenal re-work in the late 1920s, early 1930s had their receivers blued and the Turkish arsenal stamp put on the receiver and the Turkish crescent moon and sometimes the date. Sometimes the date will be present, sometimes not. Many of the parts were re-blued or had bluing added except for the bolt. These also have the Arabic/Turkish style numerals scrubbed off and the newly adopted Western arabic style of numbers and re-stamped.
Finally, the final Turkish variation of the Turkish modified Gewehr 88s would be the Turfek 1935 where the Gewehr 88 would be converted to resemble the Turkish model 1903 then anything else but still retain the same action, bolt, and magazine system of the 88/05. Many of the German proofs are removed at this point but sometimes not all. The Turkish arsenal that performed the modification will generally be stamped on the receiver along with the date with the original German marks sometimes being removed. Many of these also recieved new barrels with a bore diameter of .311 land and .323 groove but it is always safe to slug the bore.
Again, this is what I have noticed in books and what I have seen online.
My 88/05 was one of many given to the Turks and, although the exterior may look worn and used, the barrel is the original one from 1890 and looks like brand new. I am very eager to take it to the range. It is also one of the ones that was not re-worked and still retains all of the German markings and acceptance marks. Better yet, the unit marked rear band matches and it is a real plus to me. I will post a few images showing the receiver and rifle in general to give you an idea of a Gewehr 88 that was not re-worked at the Turkish arsenal level.
In the end it comes down to what the Turks were able to re-work first, probably simply grabbing the nearest Gewehr 88. They had hundreds of thousands of them in stock I bet and some were overlooked as others were picked for re-work. This would explain how you will find Gewehr 88s similar to yours and mine and others that have gone the full course of re-work to resemble a Model 1903. There is a Turfek 1935 on the EE if you ever wanted to see what one looks like. A really beautiful conversion IMO and something on my want list should my gun cabinet ever expand.
