Have a 7.5 open diff. with casting number 558912 is it a 2 or 3 series carrier?

The casting number isn't going to tell 2 from 3 series carrier.
Open it up and get the numbers that are stamped in the ring gear.

The casting number isn't going to tell 2 from 3 series carrier.
Open it up and get the numbers that are stamped in the ring gear.

That's the number off the carrier there isn't a ring gear.