1-3/8" (35mm) square end for the full shaft.
Its not rectangle, and can be rather large for smaller hand sizes.
Ash is MUCH less strong than my hickory. But its also lighter. If you are doing limited sparring and mostly training with the Helgi stuff, this should work fine for you..
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 ash. The 2 layers increase strength and reduce possibility of warping.