The very first guns did not hold the bolt back, the trigger assembly was set up to just catch the striker or hammer or what ever it was called, that apparently led to problems and the bolt hold back was an early design change. The parts catalogs do not list any parts for the earliest guns but have a note that says the only repair for them is to do a conversion to the later type action. I think holding the heaviest part back helped slow the movement down so the timing was not as critical to keep the guns from going full auto.
Last edited by GeneB; 03/05/17. Reason: added another sentence
