What year/car is it? I've seen on an old one it had a similar groove as the newer dizzy, but you couldnt get a seal into the old one as it had a ridge, it had a gasket round the top instead.
The o'ring is listed under electrical -> ignition distributor
1st you look under 'distributor versions' and figure out what number yours is, then look under ign distributor and check the o'ring or seal, and look at what seal part # is listed for your dizzy number

For example:
026 905 261 is listed for a Digifant 8v dizzy
