This U-shaped gasket is used with the unique windows on 1950 to 1951 Airstreams.  It is made of black silicone rubber.  It fits inside the metal U-shaped chanel that holds the glass.  For everything to fit correctly, you will need to use glass of the original 3/32" thickness.  3/32" glass is commonly known as "single strength."  Most good glass shops have (or can order) single strength glass.

Sold by the linear foot in continuous lengths.