I was uploading a collection of photos and measurements I'd taken for a 3-D print sample so figured I'd share a few of the basic pics for general interest.
These are the roof retractor cover retainer inserts. They appear to be numbered #9686109 regardless of type and revised in shape for later cover versions.
These help align the cover openings where the belt passes through and also holds it together tight.
I actually had one of these on my car, or a spare set, that had a lollypop stick in place. Perfect fit. I like to think it was a lazy production line worker, saving costs.
1974-1977 uses the black type. 1978-1981 use the white type. The white type will fit all, the black type will not, however, it seems easier to remove as can turn using the tabs to release. These also fit many other GM models at the time. I'm not sure the exact reason for the shape, easier to remove, but fills in part of the gap where brightly coloured caps on the seatbelt retractor mechanism and bright gold anodising, this was only visible through the centre of the gap so no need for full length, especially if had to be universal among other models that may be different.
The 1977 Firebird documents already show the white revised type even though often not what was in use. I don't have any earlier Service manuals to confirm what they say, or if it was to save costs or another model required the change before Firebirds did in 78.
Firebird 9686109 seabelt retractor cover retainers 75-77 black 78-81 white (1) by
Ben, on Flickr
Firebird 9686109 seabelt retractor cover retainers 75-77 black 78-81 white (2) by
Ben, on Flickr
74-77 retainer in 75-77 cover.
Firebird 75-77 seatbelt retractor cover retainer fit (1) by
Ben, on Flickr
Firebird 75-77 seatbelt retractor cover retainer fit (4) by
Ben, on Flickr
Firebird 75-77 seatbelt retractor cover retainer fit (3) by
Ben, on Flickr
74-77 retainer in 78-81 cover. Doesn't fit
Firebird 78-81 seatbelt retractor cover with 75-77 retainer fit (1) by
Ben, on Flickr