In post #7 here: https://www.largeformatphotography.i...r-and-adapters we can clearly see that it is a No. 3 Ilex Acme.
I heartily second Rick's recommendation that every large format photographer get 6" caliper and both a set of metric and inch thread gages. Cheap digital read-out calipers will switch between inch and metric and should be "good enough".
David "feels compelled to buy Starrett but you needn't" Lindquist
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