Rubber, probably neophrene, grommet to seal copper water tube to powerhead adaptor base on 1960's Johnson and Evinrude 3 and 4 HP models.  Brand new part from marine engine dot com - these are hard to find and they really soak you for 'em at a list price of a whopping $29.69.  Schematic in Pic #4 shows it as part #12.  Your motor won't have adequate cooling water flow if this is deteriorated and there's just no adequate substitute or MacGyer'ing that's going to work on this application.  I bought this but sold motor before installing; my loss is your gain. Priced at a bargain and shipping is free, too.  Thanks for looking.