JBH Model-54 Air Pistol  

JBH Model-54 Air Pistol.

With thanks to John G for the first two pics and text:

"JBH 54: A rare Czech underlever, 5 round-shot repeating pistol, made by Josef Brunek, Hranice, between 1954 and about 1956. Shot (4.5mm) are gravity fed into the breech from a hopper mounted above the barrel and accessed by a sliding cover."