Model 57 receiver made from model 60

[Rutgerssmith]

I recently received a model 57 in 22lr,l,s with the aluminum receiver. On taking it apart, i noticed the receiver had the same small notch that the model 60 has to retain the end of the recoil spring rod. Did they just take model 60 receivers of the line and mill them for the bolt ? Are all the alum framed 56/57s like this ? This one was cracked and I'm wondering if this was just someones clever modification or was that how they were made. I have two 56s but both with the steel receiver so I have nothing to compare it to. I know the 57 was avail in 22 mag but if the receiver cracks with just 22lr ????? were the 57Ms steel receivers ? If the mod 60 receiver was indeed used I can resurect this old girl with a new receiver.