Further info for the hell of it.
Random ebay link GM #1155059 is listed as both front and rear fenders for Firebirds in '76-78, trim to suit rear length and both with spear ends.
(Possibly '74-78, as per other common colours.)
Listed as mid-78 change to a specific shorter length rear piece with spear end.
Listed as 1979 change of rear fender strip to have the picture frame end instead of spear, tapered to match wheel arch angle.
1980-81 is listed but has new part numbers. Many had new numbers at this time, it does not say if they were the same parts or different, or mention ends used.
There is also a list of trim for different colours for 73-79 models of all GM, cut to suit and sold as Accessory Trim set, different numbers to pre-cut lengths of model-specific.
For 1980-81 they list a new set of part numbers for their GM Accessory Trim for the various colours offered, but also that they come in lengths of either 1575mm for doors or 400mm for fenders, yes, metric.
This is a common guideline to many colours also, again, if the dealers used GM stock or a trim guy to apply and follow accepted ruling is anyone's guess, and how the rivet type came to be used as well.
Firebird adhesive vinyl body mold ends by
Ben, on Flickr
Original adhesive vinyl body molds 79TA 1 by
Ben, on Flickr
Original adhesive vinyl body molds 79TA 3 by
Ben, on Flickr
1979 Formula W50 decals door edge guard body molds by
Ben, on Flickr
Firebird door vinyl body mould strip repro spear and used taper by
Ben, on Flickr
Not that the picture frame end versions were supplied to suit both sides when one end is cut off.
NOS GM Firebird rear fender body mould trim white picture frame end 1155396 by
Ben, on Flickr