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Additional information is on Sheldon Brown's Seatpost Size Database.
A 1982 730 and a 1983 720 have been reported using a stem size of 0.833" (21.15mm); a 22.2 stem was too big.
ow, one can search e for both used and new old stock (NOS) parts.
Included is a list of places where you can get additional information and where to get vintage repair parts.