Many 50s and 60s travel trailers--including Shastas and Airstreams--have Hehr series 1600 or series 4600 jalousie windows. Installed in the channel near the sill, the bottom pane of glass closes against this seal. In some cases the top pane of glass closes against it up in the header.
Because it fits well in the channel, it is held in place without adhesives.
The dart on this gasket (part that slides in the aluminum channel) is 1/8" wide.
Made of non-shrinking UV proof gray silicone and sold by the foot in continuous lengths.
Old Hehr part number 009-120.