[Jonathan S]

The Mustang vent is a little narrower than the original. The length is shorter than the length of the longest side of the original but about an inch longer than the shorter side, if that makes sense. I will post measurements later. On availability they seem pretty common on ebay. Search for
"Mustang speaker" and you should find one. From what I hear a 63 Comet vent is a better fit but they are harder to find.