There’s a certain group that will always say, “Chrysler did funny things back then”. I think there were fewer funny things than people think. Production control, even in the 60’s was pretty good and new models got new stuff. The last thing customers want is last year’s stuff. A good thing to remember is that the reliability wasn’t nearly as good back then. Some major repairs happened early in the life of some of these cars. Looking at what appears to be an “unmolested” car usually can actually be a look at a car with some significant work.
My ‘64 Sport Fury was “unmolested” when I got it, or you’d think so if I didn’t have the full maintenance record. It actually got new rings under warranty at 37000 miles in ‘67. It also got a valve job at 70000. When I took off the heads I found the casting dates were years apart. Something was swapped.
So with that anecdote as a lead-in, I feel confident saying all ‘64s had 6 bolt covers.