Originally Posted by VictorLouis

Those early flat-latches also have proprietary bolts. That's why it's not just s simple swap . The little exposed screw you see goes through the flat latch, threading into the bolt. The 'modern' thumbpiece guns have a threaded stud as part of the bolt. That's what protrudes through the side of the frame when you remove the thumbpiece. That's why they have those larger, hollow based nuts that screw down onto the protrusion.