(It's a standard-profile accessory rail. A whole world of accessories is made for them: riser blocks, hand stops, palm rests, target slings, inserts to fill the slot flush, and more. None of 'em are legal in a formal 10-metermatch (which is why many vintage match air rifles don't have a rail), but quite useful if you use the air rifle to practice for other disciplines - and huge fun to just mess around with, LOL.
Anschutz target sling with hand stop:
Free rifle palm rest, also an Anschutz component. Kinda extreme but very cool to play with:
What I use most, though, are simple target bipods. Not as a functional shooting rest, but just to aid handling the gun comfortably, without getting dings in the wood. Good to have during a long offhand session with a heavy target rifle.