Yes, they are perfectly good. For much of WWII, this is the type that was manufactured by Saginaw Steering Gear, the single largest producer of Browning .30 caliber machine guns. This was just one component in their quest to make malleable iron castings a new standard, where appropriate, to lower cost of manufacture while maintaining required performance specs. While it has always seemed a bit counterintuitive to me, some of the cast components were actually stronger than their milled steel counterparts. When new production of .30 cal Brownings was undertaken in the 1950s, the cast iron lock frame was the only version produced. There have been a fair amount of these late production, NOS lock frames available for some time.
There are three separate patterns: WWII SG type, and two variations from the 50s, though I have found no way to know which mfgs did which of those two styles. My assumption is that one is RIA and the other SAK, but none are marked that I have seen.
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