See: http://www.ply33.com/Parts/group5#309399
Part 51019 is listed as the cover screw gasket on at least PB through PF master cylinders. The cover screws themselves are 1/4-20x9/16 and on the PB through PD there is an external star lockwasher called out (but not, oddly, for the PE and PF).
Anyway I am guessing that 51019 is a fiber or rubber washer used to seal against any fluid seeping out of the bolt holes. But these are long gone on my car so I can't tell exactly what they are, their dimensions or the material they are made of. Does anyone have a master cylinder with one or more intact?
If not, maybe someone has the similar part 320094 used on 30-U and PC models...
They are soft copper washers, 1/4" bore and their outside diameter is just slightly larger than the star washer's.
Thanks!