I don't recall seeing a discussion about the fastback side panels since some came with latch cover plates and others did not.
The vast majority that I see have the covers. I ran across one today, picture below, that is like mine.
Any information as to why the two varieties?
I remember back in 1981 my driver's side was cracked. I hit the "Dearborn" swap meet and went through about 20 of them with the holes needing cover plates before I found one that didn't.
The ones without the cover might look cool, but imagine what it takes to adjust the latch in or out so that it works properly. A lot of stuff needs to come back out if you don't test it first.