When Germany occupied Belgium in the summer of 1940, the took over the FN factory complex and ordered production of the High Power pistol to continue. It was put into German service as the Pistole 640(b), and nearly 325,000 of them were made between 1940 and 1944. The first ones were simply assembled from finished Belgian contact parts, and included all the features like shoulder stock slots and 500m tangent rear sights. As the war continued, however, production was simplified. The stock slots disappeared first, then the tangent sights, then the wooden grips (replaced by bakelite) and eventually even the magazine safety was omitted. Resistance among Belgian factory workers increased as well, with deliberate sabotage in the form of incorrect heat treating, errors in fine tolerance parts, and sometimes even spending lots of time to give a very fine surface finish instead of making more pistols.

These are a particularly popular subject of collecting, and there are a lot of nuances of the production and inspection marks that are worth understanding if you want to take them seriously. I highly recommend Anthony Vanderlinden’s 2-volume book “FN Browning Pistols” for very good detail on these, as well as other FN handguns:
https://amzn.to/42Bc541

Full video on pre-war Belgian Army GP35 High Power:
https://forgottenweapons.vhx.tv/videos/belgian-gp-35-pre-war-browning-high-power-complete-rig