Haha firstly, I know nothing. I just recorded and saved pics in the past.
There's at least 2 sets on ebay that list those numbers. Prices are possibly decent for a rare item that will likely not be reproduced. Only have to see how many newer versions exist in terrible warped and crumbling condition to see these ones are excellent.
https://www.ebay.com/itm/285488706539?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=7YIat7fJRl-&sssrc=2047675&ssuid=Otl3lLdJQZG&widget_ver=artemis&media=COPYhttps://www.ebay.com/itm/276206441097?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=2QclO0DhQoe&sssrc=2047675&ssuid=Otl3lLdJQZG&widget_ver=artemis&media=COPYBeing Fisher body parts they often had different numbers to GM and many are not even listed in GM parts manuals. This may be the same here, those numbers are not in the GM parts manual but the alternative numbers of #9685872 / #9685873 are listed as 1974 only. Of course, nothing seems to come up when you search for those so the cast numbers may not be those that GM used, which won't matter unless they are in a box, people will just advertise by what they came off or what the numbers on them say.
Why are they 1-year only? Not sure, but I'll guess. 1974 was first year of roof retractors and also the roof lining board. It was last year of small rear window. Often when something new comes out it is adjusted slightly for the next year. I'm not sure if 74 was roof retractor only or dual setup with floor retractor as well, that may have meant different belts being used, different anchor points or setup in roof.
Without knowing, I'd have said that the 75-77 roof covers, while being longer, had the same hole spacing as 74. But why was it much shorter, was that a roof lining or rear window thing, or they just wanted to enlarge the covers? Not idea and don't have any good pics inside to compare.
What I do have here is a set of the 75-77 covers if you wanted to compare measurements. These are of course different to 78-81, hole locations due to turned retractor angle.
1974 set
1974 Firebird seatbelt retractor roof covers 9676030 9676031 used (3) by
Ben, on Flickr
1975-77 set. Note that they are shown on the opposite sides to above. Measurements below from this set.
75-77 Firebird seatbelt roof covers blue 1660606 1660607 (1) by
Ben, on Flickr
The screw hole spacing on the 75-77 set are inline and parallel with direction of vehicle, lined up with edge of belt, and are 4-5/8" apart.
The rear screw hole to the rear of the cover edge is also 4-5/8". This meets up with the edge of the larger rear window trim of 75-81 types. 74 type rounds off short or rear trim.
The forward screw hole to front edge is about 3-1/2".
The 74 covers are short and squatty, possibly taller but could be just the illusion of being shorter for same height. Instead of a gradual taper back they have an angled back area for clearance so perhaps the retractor shape was different to the year after.
These are all just guesses until someone knows or if you have anything there to check with. It's good to have year-correct parts, just trying to compare to the next model to see what changed and if you could use them if needed to.
