The 67-68 style back-up lights used the larger grommet, so I can only assume that there was a mix-up with the 65-66 style and a bunch got made wrong.
A simple solution is to remove the grommet from old original wiring and install on new. Or, if the whole wiring piece is good on and original light, remove the socket and reinstall on a new chrome housing. I've done this several time with nice results.