Here's another old one-hander that I happen to have lying around the house, a BSF S20 M pistol from the 1960's.
BSF's were imported by Air Rifle Headquarters, Hy-Score, and later Beeman; and the S20, along with the model 55 sporter rifle, were very popular. They must have been well-loved, too, as they are notoriously hard to find in good condition! The classic one-piece grip/frame is a classically German detail, and of a very distinctive shape on the S20.
I admit to a special affection for this particular pistol. A good friend of mine had an S20 when I was middle-school age--the first quality European airgun I'd ever seen or used. Its nifty looks, quality materials, grown-up heft, and ease of use were quite the revelation at the time...you could say, I'm still trying to recover! The sucker shot hard too--the S20 had a justifiable reputation as the hardest-shooting air pistol available for many years. This oldster still shoots well over 400 fps.
The attractive enameled brass medallion shows this is a house-brand gun, but BSF airguns were often marked for individual distributors, and thus are also found bearing the "Burgo," "Wischo," "Bavaria," or "Hy-Score" names.
I don't know much about the history of BSF airguns, but it's apparent there were three pretty common variations of the S20. The basic gun was similar to mine but had a simple, fixed-elevation, drift-adjustable rear sight in a transverse dovetail on top of the receiver tube. The S20 M ("Match"), added a click-adjusting rear sight built into an extension plug at the rear of the receiver (the plug is milled on this gun, but later ones were a well-finished casting of slightly curvier appearance). A later version of the M, seen fairly often in the US, added a fancier grip with a small thumb rest and palm shelf, but the basic grip is more attractive to my eye, and more comfortable to my hand.
Above are closeups of the rear sight and its mounting plug. The vertical adjustment is a nice clicking knob, while windage is an odd slider gizmo that requires loosening a screw. Note the small screw underneath, which is the--rather unpredictable--sear engagement adjuster.
The tapered barrel is a nice styling touch. The front bead sight is very precise, but can do some damage to your hand when cocking this stiffly-sprung gun. The later M version, with the revised grip, also had a large snap-on sight hood to avoid this problem.
The action is a little unusual, being based on a small-sized rifle. Most break-barrel air pistols have a compact trigger mechanism whose sear grabs the outside of the piston. On the S20, the trigger is "bullpupped," i.e. moved well forward of the sear, which must reach way back to engage a central piston rod at the rear of the action. It all looks a little weird, with the sear bar actually hanging out the back of the wood, but works well enough.
I really like the handling of this old pistol. It's long, but like most of the old BSF's, the S20 is quite slim and efficient. Light and nicely balanced, with fine power and good accuracy, it's a real period piece that can still hold its own performance-wise. Highly recommended!