I've put new seat rubbers on 5 so far (two 88's, s77,s55 and 35 combine) and haven't had any trouble with rotten rubber, just stiff. the stiffest was an early style ridemaster seat, the later ones seem somewhat more flexible and the single torsion rubber on a super 55 is the softest of all, yet still firm. I assume they are offshore manufactured.  (got them from a retailer in Ontario via Steiner Trac. parts. THANK'S)  They will flex more over time but with the limited use some of these 'oldies' get that can be awhile.