From what I have read, the design of those stamped/pressed ridges plus the three "mini-ribs" on the top (all used to stiffen the sheet metal from flexing) appeared at about the same time as the stamped-and-folded sheet metal receivers, which would not have the same rigidity as a machined receiver.
Raised ridges are common in magazines and are there for stiffening/anti-flex purposes. Raised ridges, folds, and crimps can allow the use of a lighter gage sheet metal to achieve the same strength as a thicker, flat sheet.
Those protrusions have a structural function and I have seen them on various models. When assembled, those top covers add some rigidity to the firearm unit. They are pretty well "fixed" front and rear and the overlapping "lips" on the sides also help. DDR
Raised ridges are common in magazines and are there for stiffening/anti-flex purposes. Raised ridges, folds, and crimps can allow the use of a lighter gage sheet metal to achieve the same strength as a thicker, flat sheet.
Those protrusions have a structural function and I have seen them on various models. When assembled, those top covers add some rigidity to the firearm unit. They are pretty well "fixed" front and rear and the overlapping "lips" on the sides also help. DDR
