Prototype Receiver with helical-cut slots to accept barrel lugs in the form of “interrupted threads.”
There is no ejector in the frame.

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.