American Eagle Telescoping Stock Holster

[Maestro]

According to "Lugers at Random" by Charles Kenyon, Jr, this is a

U.S. patented "IDEAL" telescoping holster stock: Depressing the release button on the top frame rod permits the tang portion to be extended (as shown in this picture), or collapsed for carrying. Note the belt hook on the left side. The holster portion is soft leather; the stock is marked "U.S. PATENT PENDING".

[Maestro]

furthermore:

The attaching device consists of twin hooked tangs which fit into a special set of grips; these grips have a smooth walnut face and are metal backed.