Tlhe Inglis High Power with an original Inglis(Canadian) shoulder stock/holster has been declared a Curio and Relic By BATFE and has been removed from the NFA reglulations as a short barreled rifle(Look it up on NFA Curio and Relic list).
The shoulder stock must be an original Canadian manufactured stock produced by Small Arms, Limited in 1944 or 1945 under the Chinese contract. Reproduction shoulder stocks are not legal when attached to the pistol(per BATFE opinion letter in 1998).
Original Canadian manufactured stocks were made of Walnut and marked S.A. LTD. and 1944 or 1945 on the right side of the grip and had the Canadian Government ownership or proof mark consisting of a "C" and Broad Arrow inside an oval on top of the grip forward of the comb. Only those shoulder stocks imported into the US are marked "MADE IN CANADA" on the back below the hinge plate.
Belgian manufactured High Powers with original shoulder stock/holsters are also exempt from the NFA rules if the stock is original and the pistol was manufactured pre-WWII or under German occupation, or was one of the special contract to Oman. Again, the shoulder stock must be original manufacture to the time frame of the pistol to be legal. Beware of reproductions.
I have seen Inglis High Power with shoulder stocks/holsters offered on Gunbroker and other internet sales sites for $900.0 to $2500.00. I suspect that there are fewer Belgian High Powers with shoulder stocks than Inglis High Powers with shoulder stocks and therefore would expect a Belgian set to be priced appropiately higher.