I've just found that one of my 'Firebird' spare mirrors is indeed not from a Firebird. It has the squared-off base and not the tapered end to suit the door side.
This on is late-'77 dated, but still uses the Firebird style mounting bracket rather than twin studs and taller support. It also still uses the Firebird type toggle knob.
Anyone here able to identify what it may have come off? I have shown it with a bare casting Firebird mirror support. I have a couple like this but the other is from a 1973 vehicle and X-bodies, as far as I know, still used the Firebird mount and toggle at that point, changing to stud mounting after about '75, from what I can see.
1977 LH remote mirror X-body support (5) by
Ben, on Flickr
LH remote mirror 77 X-body square support vs 78 Firebuird tapered support (1) by
Ben, on Flickr
LH remote mirror 77 X-body square support vs 78 Firebuird tapered support (5) by
Ben, on Flickr
LH remote mirror 77 X-body square support vs 78 Firebuird tapered support (2) by
Ben, on Flickr
LH remote mirror 77 X-body square support vs 78 Firebuird tapered support (4) by
Ben, on Flickr
The LH support is 52842. I also have a RH support that matches being 52832, shown with a bare Firebird support casting leaning against it.
1977 LH remote mirror X-body support (4) by
Ben, on Flickr
RH mirror supports 52832 X-body 29845 F-body (1) by
Ben, on Flickr