Headrest Question

I've seen late 60's/early 70's GM headrests with 2 round supports each, or one flat center support. Anyone know what determined which type was used? Is there some consistency by year or body style?

 

I thought the two round support meant convertible and the single flat bar was the coupe/hardtop.

 

FWIW, I thought the 2 bar is a bucket seat version while the flat single is a bench seat.
Troy

 

Troy, u are correct.

 

Thank you!