No answer on Norms question of : What years did they use the same seat, and did the other GM wagons have the same seat and hardware? That way the junkyard crawlers like myself, can keep an eye out for you.
From what I've read, It appears that 65-70 chevy wagon 3rd seats will interchange. I'm still researching to see if the BOP 3rd seats are the same. I've done some basic searching and apparently the 3rd seat is a scarce find.
Great, I'll need all the help I can get finding a 3rd seat! I've found more 59-64 3rd seats than anything, and they do ask a salty price for rusty frames with no padding.