After more research and going directly to the source (Enrico) who remembered exactly what battery was in the car when it was delivered (would you expect anything less from him? [img]<<GRAEMLIN_URL>>/grin.gif[/img] ).
This is what I am now searching for. It would probably be easier to find that '65 Z16 convertible before I find one of these old batteries, but here goes:
I am looking for an original (NOS, used, dead or alive) Delco X Energizer Maintenance Free battery. These were the optional maintenance free battery that were offered in Pontiacs from 1971 to 1975. In 1976 they were replaced by the red, white, and blue Delco Freedom Battery.
No one reproduces this battery as it is just too obscure. Here's a couple photos combined from a 1971 and a 1975 brochure:
