Squareback alt with one normal and one large spade connector

I am bringing a neglected 68 Formula S 383 back to life and have run into a few electrical problems. When I got the car, it had an upgraded electronic ignition and an electronic voltage regulator but an older style roundback single field alternator. I know that this will not work so I bought a new squareback dual field alternator. The alt has two different spade connectors, one large and one small. I found out that the large connector is grounded after I melted a jumper wire when I tested it in the car. I assume that this squareback has actually been rebuilt to work with a single field system. I will return it tomorrow, but what year of car should I get the alternator from?

Thanks for any help,