You mean a BUCO BATES mirror ? who also did head lights, seats etc
They would have been nickel and dime mirrors back in the day, expensive if you can find one (NOS) these days.
Who knows, maybe they were a cut down long stem mirror ?
There are contemporary copies that look 'OK but the mirror OD might be a problem at 4 inches.
Glad you got it sorted.
I have been looking for long stem BATES mirrors for my Eldorado for quite a long time but they are hard to find.
The modern versions just don't quite look right and have cheesy clamps :rohan
This is a period under bar mirror, its not bad for its age, from the era where even a cheap mirror had some quality.