The surface of the frame ears of the original pistols was hardened (so were those of the post war Mauser Parabellums). Aimco/SPM/Orimar probably never bothered to copy that feature.
The holdopen has to 'catch' behind a shallow cutout in the bottom of the breech block. I would check that area for wear as well, I can image that, because of the relative softness of the stainless steel, the breech block cutout will also wear, allowing the breech block to 'hop over' the hold open. A weak hold open spring will also allow this.
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