Prototype Receiver

Production Receiver

Remington 51 Receivers
So far as I am aware, no one before or since has used helical lugs and grooves to fit a barrel to a receiver.  The fitment is very precise:  you must get the slide aligned to exactly the correct position, yet once aligned the barrel turns into the grooves with minimal effort (though it can be difficult to get a good grip on a smooth, oiled barrel).  Note that the original receiver design had no ejector.  The ejector was on the magazine.  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 similar ejector.

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