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.