I had/have something very similar, inherited from my grandfather, but so far as I remember each threaded hole had a smaller intersecting hole on either side, to give two cutting edges,, it was a 'die plate' for cutting small threads. Yours could be a thread gauge, but then I would expect the holes to be marked in some way, or maybe it's intended for 'rolling' threads?
I was surprised that your attachment got through, they are usually blocked.
Cheers Tim
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