Foreign and Captured Weapons in German Service
The German army was exceedingly frugal when it came to equipping troops with firearms and other equipment. Huge stores of foreign war material were captured as countries were annexed, conquered, or as enemy troops were defeated. Some of these weapons were reissued to German soldiers as substitute standards. In these cases, the firearm was given a new German designation such as the Czechoslovakian machine gun TK vz.37 being renamed the MG 37 (t). The (t) designated the country of origin (Tschechoslowakei). The sort of troops issued foreign guns tended to be troops with a more second rate nature. They were issued the static guard troops especially. When General Mahlman took over the command of the 353rd Infanterie Division one of his chief complaints was the number of different calibers of ammunition he needed to obtain for all the foreign weapons in service within the division.
In the case of these photos, it is impossible to tell without more context if the soldiers were issued these guns, using them without orders, posing with them as curiosities, collecting them for disposal, or anything else. We cannot merely assume that the fact that we see a soldier holding a flintlock musket the German army was in the habit of issuing them.