I ran across this tid bit on Restoration Battery's website.
http://www.restorationbattery.com/gmproducts.html Scroll down to the Chevrolet section. They list it like this.
1963 - 1966 6 Cyclinder 283 & 307 Without A/C
22F DC12
1963 - 1965 ALL with A/C + 327 & 409
558 DC12
Anybody have any more information on this? The only other bit I can come up with is this picture from the Motor Trend review of the 64 Impala (with 327) which seems to agree with Restoration Battery's recommendation of a back cap with light colored letters.
![[Linked Image from thecairns.com]](http://www.thecairns.com/Impala/MT-Engine.jpg)