Many thanks to everyone who responded. This club is the best!
I called OPGI and am exchanging the mirror for part PZ00438, a 10 1/8" mirror which they list as for 1957 - 1966. But the comments confirm that this is really correct through 1968.
Bob, I totally agree about one-size-fits-all restoration parts. And it adds another degree of difficulty, when the car you are working on is not original, or is missing parts.
I'd rather get a new mirror than re-silver, because the back looks like it's been submerged for the last 45 years. Like the rest of my car, it's not pretty. But that's changing, slowly. This is actually the first "restoration" part I've bought - up until now I've been using used, or replacement mechanical parts.
Cheers
Bob