The external regulator alternators are smaller in diameter than the 10-SI, and have a 2-wire square plug that inserts into the back of the housing. The internal regulated 10-SI has a flat rectangular 2-wire plug on the outer perimeter of the stator housing. There is an aftermarket regulator available for the early alternator that attaches to the back of the alternator, plugs into the square receptacle, and eliminates the need for an external regulator. I used several of those units while doing 12 volt conversions on farm equipment back in the early 80's.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!