Green’s Dictionary of Slang

a potato, its surface jagged with the edges of safety razor blades.

1950		J.D. Carr Below Suspicion 132: ‘Use the moleys when you catch ’im.’ The moley was an ordinary potato, its surface jagged with the edges of safety-razor blades. They ground it into your face, [and] twisted it .
1959		Spectator 6 Mar. 314/1: I suppose if I go on criticising him I shall end up by having the boys with the moleys call on me one dark night .