My general impression is that the third seat for the Burbs of any given year is the same as the rear seat of the Blazers for those years. I am reasonably sure that said seats are the same for '73 through '77, when the pan changed. These seats bolt down on all four corners.
After that I'm a little vague. My '85 CUCV had the bolt-in-front-but-fold-n-flip seat, but CUCV's are a little odd as they're all basically '83-84 trucks as far as parts go.
Correspondingly, my '90 Burb had the fully removable, i.e. four latch point, third seat.
I would *assume* (and we know what that spells) that the break is somewhere between '85 and '90.
I would also assume that to make the four-latch seat fit into your K5 you'd need to cut out the latch sections from the floor and weld them into your truck, which seems like waaay too much work when you should be able to scrounge the right seat for your truck.
HTH.
-- A