Help identify this breaker

It's an interesting type of circuit breaker, a Hydraulic-Magnetic. See the catalog here.

Anyway, the third pin is actually for a trip coil. Which is a coil when energized, will trip the circuit breaker. Ther are most often found as clip-on units for rail circuit breakers. But this one has it integrated.

Different catalog.

See the diagram on the lower-left of page 19 of this document. Your breaker would appear to be a "dual rating" type, in which an overload on either of the B or C ("load") terminals will open the circuit to both.

Terminal A would be the "line" connection.