Crosman Model-677 "Plink-O-Matic"
Crosman Model-677 "Plink-O-Matic".
Courtesy of Leonardj.
The shuttle that feeds the ammunition is narrowed, to allow the feeding of the BB's, and the filler plate, as indicated by the arrow in the left hand side of the picture, is used to fill the gap that would have been left between the shuttle and the frame.
The arrow on the right side of the picture points to the sleeve that is placed inside the magazine to reduce the inside diameter to the appropriate size for use with BBs. Access for loading BBs is gained as the magazine follower is drawn back, and as the locking lever is moved into it's retaining slot, it rotates the filler tube just enough to open the loading port.
This pistol was spotted on Ebay by D T Fletcher: