I wouldn't say rare,
think how many Sanitronic 50's were cranked out of the factory in 1964, and the Dual 50 ran from 64 to 67 (Replacing the Sanitronic VII and seceded by the Sanitronic 80). if it ran for 3 years, that would logically make 1/3 of all Sanitronic 50's having the 1964 Head-lamp-in-cap. 1/3 of All Sanitronic 50's, not rare, but nice to have.