Looks like aftermarket... There were tons of different aftermarket mirror styles available in the '50s as outside mirrors from the factory were optional. Many dealers would buy aftermarket as they were generally less money, charge factory price and pocket the difference.
Appears to be a fender mount rearview mirror, no idea of an original OEM application that looks like that. Agree it seems aftermarket.
I'd agree with aftermarket. If you could dig up an old JC Whitney catalog out of the late 50's or the 60's they had pages of mirrors to choose from along with pages of hood ornaments.