If I had a '66 300 I'd be getting several, everyone I know who has one has a tale of woe associated with them.
These are almost as likely to be broken as those $%*& turn indicator lights for '67 Imps. Even an NOS set will now be 40-50 year old brittle plastic.
These were at one time available from a guy in Australia. I think his name was George Laurie, but doesn't seem to be around anymore. Seller for these seems to be in Australia as well, wonder if there's any relation?