So, what kind of seat is this? The other GLs I've seen had a fixed
back on the third seat, whether they were cloth or vinyl.

I have not seen a GL seat before with a seat back latch and pivots,
such that the seat back drops back over the rear deck and makes a bed.
This one originally had a narrow way-back deck cushion as well,
similar to, but less deep front-to back, than a Westy deck lid
cushion. The whole setup makes a short, but almost queen-width, bed.
The floor brackets on this one are intended to fit against the
interior slope of the rear axle/engine area.

Jordanvw:
hate to tell you, but that's not a Z bed.    that's a  standard
passenger
van GL rear bench seat.  does not make a bed.

just a fyi.