I believe the "C" designates it as a sporting model. Likely came from Perazzi with a non-Monte Carlo sporting stock, a shorter flatter shooting rib and as an o/u only gun. Maybe adjustable or selectable trigger? Assuming it's a MX3C, I believe this had a round mono block as opposed to the square block of the MX3 Special.
A MX3 Special is a trap model that came as a top single and/or combo. Has a square mono block like the rest of the MX family (mx8,mx15, mx2000, MX10 etc.) which means barrels and receivers are interchangeable with minimal (if any) fitting. Has a slightly taller rib than a MX8.