Internal DWM Factory Stock

[Imperial Arms]

When I was visiting a collector in Europe, he showed me this special stock which fascinated my interest. Apparently, it was a stock used inside the DWM factory to quickly fire pistols off the rack without wasting time attaching a proper stock. The stock connects with a flat hook behind the laynard loop and meets the contour of the rear gripstrap of the pistol.

Can anybody guess when this stock was probably used in the factory, and how much would it be worth?

Cheers,
Albert