Prototype Receiver with helical-cut slots to accept barrel lugs in the form of “interrupted threads.” There is no ejector in the
Production Receiver with two holes to accept the semi-headed pin that holds the barrel in place.
In production receivers, a pivoting ejector was used--it pivoted down to allow the slide to go on the receiver, but was pushed up into position
by the top of the magazine. The Colt Mustang uses a somewhat similar ejector.
This patent drawing shows the helical barrel lugs. It does not show the full number used on production guns, but the intent is clear.