Unfortunately, My 1897 is a "E" prefix gun and I have never examined an earlier gun in enough detail to say what exactly is supposed to go in the second hole.

I would suspect that it would be for a block similar on that of the later gun, so that the smaller pin in front can be beat up by the rearward moving rim of the shell and is replaceable, and the block prevents it from getting bent enough so that it can't be removed.
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