Dave,
Some adjustable cheek rests were a factory option. The photo below is from a Von Lengerke & Detmold Import Record book that is in the custodianship of Griffin & Howe. The M-S rifles and carbines were received in December 1920 from Johann Springer's Erben of Vienna, Austria. The quantities received are broken down by caliber, barrel length, make of telescopic sight, and the presence of an adjustable cheek rest. The serial numbers are on the next page. Do you believe that your M-S has the factory cheek rest? Not all VL&D Mannlicher's were so marked. In one instance, VL&D ordered 100 Mannlichers but had their name engraved on only 80 of them. If you will send me the serial number I'll see if your gun is in the VL&D records.
research@griffinhowe.com 
Compliments of Griffin & Howe, Inc.
