What's shown in that EPC screen shot is NOT the same as the 1987 300D pump. The rear gasket (for the governor cover, below the ELR) on an M-type pump is p/n 001-074-43-80.

What specific pump are you trying to get a gasket for? An M-pump on a 617? If so, the 001-074-43-80 should work, and it is available.