Here's a strange one. I pulled the starter off the 68 motor and the number on the case is 1107709 4. B. 1 When I google it I get replacement starters for Catapiller or old international trucks or GM applications but it doesn't look like a Pontiac starter. Maybe they used a different case when it was rebuilt?
That number is a "10MT" starter so it is the correct model starter,just not the correct part number for your car. It will and did work fine I'm sure as these model starters were used for a number of years on a variety of applications. I'd recommend finding a number correct starter, used and rebuild it. They are extremely easy to rebuild.