Jim,
Your info on the voltage regulator condensers (capacitor) matches what I have seen. I have a used C3SA-18832-A with the same markings you describe that I believe came from a 65 Mustang. It's actually a T-Bird part number, used on 63-65 birds, but was also apparently used on 64.5-65 Mustangs. The one on my late March 66 San Jose built car is a C6OA-18832-A FoMoCo but can't be sure if it is original. I also have a NOS C6OA-18832-A Autolite like you describe. I've also seen one or two C5OA FoMoCo's on ebay but they seem very rare. The C3SA and both C6OA's I have are physically identical.
Does anyone have any info on the production windows for these different engineering numbers?
Dave