After considerable searching, this worked perfectly. Had to buy a 100' roll. used 20'. Selling the rest. PM if interested in buying the rest.
That's correct. They used "vinyl glazing channels", same as camper trailers of the era.
Robert at VintageTrailerGaskets.com has been very helpful
Steve at VintageTrailerSupply.com turned me on to DK Hardware (search: vinyl glazing channel)
Still trying to pin down the exact one, but...