Hey Tom,
I'm a Metuchen Sprint owner, too.
If your buck tag is the original one that came with the car, then 65A would be a standard bucket seat interior. 65C was the bench seat. You may want to peel the carpet back a little and see if there are any holes where a console would have been screwed down. The Sprint package would have included a console. I know of no Sprints with bench seats.
It's believed that Sprint production began at all plants at the beginning of January 1966, although there have been some reports of Sprints as early as December and November.
Dave's right about the possible build sheet location behind the speedometer. That's where I found mine. I had to remove the speedometer. It was folded up and taped to some of the wiring harness there with black cloth electrical tape. Hard to spot without removing the speedometer. Other places you might look include behind the door panels, under the seats, or under the carpeting.
I'm just beginning to put together a blog where Sprint Owners can find information unique to these Mustangs. You can find it at:
http://66sprint200.blogspot.com/My intention is to open it up to all Sprint owners. If anyone with a Sprint has some pictures of their car, or other Sprint info they'd like to share, just let me know and I'll upload it.
Pete