Ok, I got a little more info from Joe Lucia (NCRS). Apparently these were used from late 1964 to early 1965 across Chevrolet models and probably other GM car lines as well.
However, 3850950 was never available as a SERVICE part. Kits like this (especially with the kind of package seen in the eBay item referenced above), are never sold in service, so more than likely these would have come in the car for the lot boy to attach when the car was delivered to the dealer.
Ed