These are the post-64 Classic era parts you would need to build a .375 H&H length rifle. These are OEM parts from a donor action.

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The code on the side of the magazine box is 1110. It is very similar to an E box, except the feedlips tip in (on an E box, they do not.) I think the E box is a better match with cases like the WBY family that have wider diameter shoulders, while this box will give better feeding with the more tapered H&H family. The follower is longer than typical as well

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It holds 3 375H&H or .416 Rem down, and works with factory bottom metal

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$60 plus shipping.