Gael, it seems you've self-discovered your answer. Mind if I hijack this thread?
We're in the process of refurbishing the windows on our 1968
Safari. The spousal unit seems to have a grip on the FUNCTIONAL WINDOWS, the curved ones that open and close, and this question does not relate to those. THIS RELATES TO THE FLAT, FIXED PANES THAT ARE BELOW THE WINDOWS THAT OPEN AND CLOSE.
These appear to rest against a 360 gasket installed on the interior side of the window frame ("the first gasket") and seem to be held in place with another 360 interior gasket ("the second gasket") that is installed on the interior side of the glass after it's placed onto the first gasket. Vintage Airstream recommended, and we purchased, three things for the project: a second gasket that looked like the first pic below. They also supplied a bulb gasket (2nd pic) and some butyl tape.
When I pulled out the first pane, I found the old gasket on the exterior side (the first gasket) to be a simple piece of flat rubber, badly decomposed, third pic below. Is this supposed to be replaced by the bulb gasket? If so, I don't think there's enough room. I mean, if I glue the bulb gasket to the window frame and put the glass on top of it, I don't think there'll be enough room to insert the second gasket into the groove to hold the glass in place - and seal out moisture. Or, does the butyl tape go onto the frame as the "first gasket"? That would not only seal it, it would do a pretty fair job of holding the glass in place.
I'm mindful of the community's criticism of bulb-type gaskets.
Jay & Lisa