There's 2 types of remote mirrors out there over the years, with different size holes through the support casting. This is likely a case of 2 different suppliers since it was found for all years. The one will the small hole had the cable remote assembled in place and could not be removed (#52842). The larger hole version could be assembled and fed through, and likewise, removed again for painting and swapping internals (#52822).
Some more random trivia of these 2 different castings, they didn't even have the same base size or length. One would fit on the gasket, the other inside the gasket. Anyones guess how they fit any of the gaskets sold now as the same for both both, Firebird and Camaro.
Has anyone successfully opened up the cable hole to a larger size to allow it to be withdrawn? Some quality Dremel time perhaps. I have several mirrors for a spares but a few have nice glass and nice chrome remote toggles, which may or may not be better used elsewhere.
I also have a mirror that has non-dated glass, and the internals are different. Has a JC casting, did JC Whitney offer these? Also has a shorter mirror pivot mount that only has 2 of the 3 mounting holes. It will fit in an earlier mirror head, the earlier glass assembly won't fit unless the 3rd hole is cut off. Anyone seen one of these? I've seen them on ebay without realising I had one, also have extra unused mounting holes behind the glass.
Actually, I could even alter this thread to What year is your mirror/glass and what version is it? It would interesting to know if there was an overlap or more an Early vs Late thing. From my own limited experience there were 2 mirrors heads, small cable hole vs large for later years, and 2 supports, small cable hole vs large cable hole. These could also be mixed up as I have on a '77 set where the head has large hole but support has small hole (cable can't be removed), I have a '76 set that has small head hole, large support hole (cable can't be removed), 2 x 78 sets and a 1980 set which had both large holes so can be removed. I had a couple of others but didn't record the hole or mirror dates, cleaned-up and resold.
Rambling, since it's impossible to discuss or ask a simple question on the length of a piece of mirror string without taking into account the different lengths and that none are on mirrors anyway. Pics help?!