1966 Barracuda trim/weather stripping
Try Bethel's Goat Farm in San Jose, it sounds weird, but if you call them, they'll help you out with the part #, also try rubbertherightway, super helpful and friendly. I would ask about how to replace the gasket yourself, they gave me some tips how to replace the locking strip on mine, yeah, it really all comes down to soap & water, but I hear the glass is heavy, that$ no ordinary back window!
I would watch a few YouTube videos of shops replacing glass and seals on vintage/classic cars, the kind that are time-lapsed so you can catch every step.