The "bulky-looking" mirror on the left came with the car when I bought it. The one on the right is a coded original that I bought to add to the passenger side (came as a pair, so they'll match). The bulky one on the left doesn't appear in any pictures of Birds I can find online. They all seem to have the slimmer-looking mirror on the right.
Is it possible that the bulky mirror is from a tempest or catalina or something? Maybe it got knocked off and was replaced by whatever they could find?
Is it a super rare mirror added to only Firebirds built in April of 68? ( heh heh).