the pistol was very unusual and is often referred to as a "veterinary gun," used to put large animals down (e.g. cattle). These pistols have an integrated silencer/suppresser so as not to disturb other animals. They also require pulling back the slide to chamber a round each time a bullet is fired. More specifically, it is thought to be a "Station Six 9," which is a 9mm weapon with origins in WWII covert operations. Being a USMC vet, I thought that I knew quite a bit about rifles and pistols, but this was a new one to me.
https://bt-usa.com/products/station-six-9/