Distributor O ring/ gasket part #

Looks like I may have an oil leak around my distributor . I have a 1966 273 block, and a MP electronic distributor. The distributor has the groove for the O ring (which I'm using). Is this right ( I sure would think so)? Or should I be using a paper gasket ?
I'm having problems finding part numbers for the O ring/ and also numbers for the paper gasket.