Back in the early '70's, there were still quite a few '55 Customlines around here and I always looked at them and noticed most of them had the wind-up clocks in them. Most of the Mainline series cars I saw in junkyards just had the plastic delete plate and the Fairlane series cars mostly had the electric clocks.
I always wondered if Fomoco had a Standard like this which could only be changed by a special order (having a electric clock in a Customline as a special order). I did see a few variances, but scant few.
I've had bad experience with one of the popular instrument rebuilder/restorers. I had my '55 electric Westclox converted to quartz movement and gave bold instruction that it should be delivered to operate on 6-volt positive ground electrical system. It lasted just past their limited warranty and died. I ain't playin that game with nobody. It makes me angry and disappointed to spend $110.00 for nothing.