1-3/8" (35mm) square end for the head, and rectangle grip on the back end.
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It will fit the larger Helgi True History Shop Polehammer Heads.
I believe there are 2 sizes, so be sure to check that its 1-3/8" square.
It is 2 layered laminated impact grade hickory. The 2 layers increase strength and reduce possibility of warping.