Blackburn makes Model 70 bottom metal in stainless. You didn't say if yours is pre or post 1964. They definitely have stainless for 416 Rem which probably would work with 375 with slight modification (may not be needed). 416 box is long enough but it has slightly wider shoulder. Both have same rim diameter.
But you won't get it for $250! Add two or three hundred. And if you think that's expensive, price Sunny Hill bottom metal! I used 416 Rem Blackburn bottom metal on my 404J on 98 Mauser project. Required no modification. I did have to clean up some machining marks before bluing but it turned out very nice. Sunny Hill would have been ready to drop in but $300 more.
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