This premium butyl glass bedding tape is 1/4" wide and 1/16" thick.  Its small dimensions make it very hard to find in local stores.

It works perfectly placed between the glass and the aluminum front frame of the window on Hehr windows used on most Airstreams from 1958 to 1965.  Click on MORE PHOTOS to see a cross-section photo of the detail of a Hehr Hall-Mark 12 window used from 1961-1965 on Airstreams.  Other awning-style windows use it in a similar way.

This is a 100% solid crosslinked polyisobutylene, preformed sealant.  It comes on a 50' spool.

Since it is self-adhesive, it requires no additional adhesives to attach.  Just place on the window frame and set the glass on it.

Color is black, but the tape will be entirely hidden behind a glazing strip on most windows, so color does not matter.

It is made by Tremco, the manufactuer of Vulkem.