yeah i had a late 78 750 that could not use the O ring chain but the rubber plug issue is a 550 issue the 750 issue had more to do with the seal boss
I thought the 78 750 came with a 630 ORing chain as standard equipment?
And the 78's have a longer output shaft as well, 10mm wider, they also have a 10mm wider sprocket carrier for the rear.