S. Hunter,
The Borden connector is the small, round detail piece on Boba's right cheek. In M_S's pics, you can see a little circular depression where it will be attached.
It's called a Borden connector because. . . well, that's what it is! It's a piece manufactured by the Borden Company for use in dental offices. It's the piece that connects the "drill head" to an air line.
A lot of folks have gotten real Borden connectors to use on their helmets. The real ones are brass (if I recall correctly) and some people drill a matching hole and install it from the inside, or cut a slice off the connector and glue it onto the outside of the helmet.
Eric