I remember that there was discussion of one place that had them, but the quality was not up to people's expectations. I think that is one of those listed below. I looked for NOS, but no luck so far. Can't say that a 40+ year old rubber part sounds great anyway...
Suppliers that do not have them are;
Robert's
Metro
Andy B
Steele
So far, I've found these...
http://www.restorationspecialties.com/dodgetruck.htm
70213455 VENT WINDOW AND DIVISION POST SEAL $140.00
http://www.patwalshrestorations.com/DOD_VAN-PU_64-70_ERA.html
VENT FRAME SEALS
VENT WINDOW FRAMES REFURBISHMENT KIT
KARR Rubber Manufacturing
133 Lomita St.
El Segundo, CA 90245
Phone: (310) 322-1993 or 1-800-955-5277
Talked to John, and he said they use high quality EPDM. The seal is in 2 parts.
He mentioned an early and late, but said the exact break is not mid 66, they have had 64 and 65s with the late style.
He said that if the pivot point is about a quarter of the way from the top, it's early. If it's about a third of the way down, it's late.
If anyone has feedback on these, please let me know. I want a good seal the first time. Seems like others could use this too!