Kevin is referring to 3 different parts, each with an important role to play in keeping your 1961
Hehr Hall-Mark 12 windows dry. The Glazing Strip is a rubber or plastic strip that is cut at a 45 degree miter in the corners. It goes on the outside of the glass and deflects water so it does not go around the edges of the glass where it meets the frame. On Hall-Mark 12 windows, this was originally vinyl plastic and was later available as a vinyl rubber. For years, we have sold it as silicone rubber. Now, we are going back to vinyl plastic for originality sake. It prevents the frame from acting as a gutter on the lower edge.
The second item mentioned is glazing tape. This is what you bed your glass in when you put in new glass. No need to change it out if you are not removing the glass from the frame. It is the single most important seal against water and should be applied very carefully with tight corners.
The third item is called the Bulb Seal. This goes in a track around the perimeter of the screen. It is the gasket against which the moving sash (called the front frame) closes. It provides a water and air seal. The original vinyl rubber bulb is often found missing or deformed completely (as Andy mentioned). We decided to reproduce it in silicone rubber to avoid the deformation issue.
If you have a 1958
Airstream, it is unlikely you have a Hall-Mark 12. It is more likely you have Hehr Clear-View windows and possible a combination of Clear-View (or Clearview) windows and Hehr Standard windows. If you have Hehr Standard windows none of the above applies. If you have Clear-View windows, almost everything is the same except as for the Hall-Mark 12 windows except that the glazing strip was originally vinyl rubber and the replacement is silicone rubber.
If you have any additional questions or issues about early 1960s windows, let me know.