As a note, the headresets on bench seats are actually flexible plastic, with an imprinted design to make them look like they are vinyl covered. I like the following seat set (looks like a taxi setup), other than its vinyl covered headrests:
Regarding the seam on the blumo moving around (on the back of the front seat)
It appears to be just sloppy tolerances by Ma Mopar.
I believe that all seat styles (for a given body type, at least for a given color) had the rear seam in the same place. The parts catalog has 6 pages on seat styles for the Monaco. pic 1 is an example.
I believe this page shows that the seat styles shown on that page were available only on DH 23-41-43
DH is custom. 41 is a 4-door sedan.
Then if you continue in the catalog to pic 2
it shows that for a 4-door sedan, for a particular color, part (2) is the same for all seat styles. I think this shows that for a given body style, for a given color, regardless of the style shown on the front, the part for the upper long horizontal strip on the back was the same. So the seam moving around is just tolerances, not an indication of a different seat style on the front.
At least I think that's the case; I didn't spend a lot of time on it. Just throwing it out there. Sadly, the parts catalog didn't show seat styles for police or taxi body styles.