After WWI, Turkey obtained a very large number of German Kar 98a (aka 98AZ) carbines; made originally to be light weight, those rifles have the standard Model 98 action but with small receiver rings to reduce weight. That was the origin of the so-called "small ring Turkish Mauser". In the late 1930's, the Turks rebuilt and rehabbed thousands of their rifles of varying models and ages to present a uniform appearance, and all those were lumped under the designation "Model 1938", which is, needless to say, a bit confusing to both collectors and gunsmiths.
The Turks did not actually make any Mauser receivers, but the arsenal at Kirikkale did have the capability of doing extensive rework, re-barrelling and the like. They also usually removed the original markings and replaced them with Turkish markings.
Jim