Can't answer the question about the diff lock, but the rear axle is neither cast iron or stamped steel. I am relatively certain that it is made of cold rolled bar stock that may have been hardened after machining. Why do you ask????? Cast iron would snap under stress and stamped steel would twist under stress.
BX1500 has a diff lock, it's small pedal on left side, next to parking brake release. It's at front edge of deck, not at back where I've found most others.