This fat knobby mountainish tire tire was designed by Kirk Pacenti, made in Japan by Panaracer.  It's listed as 51mm wide and fits the Bombadil just fine, perfectly.  It's light (535 grams) and has shallow, widely spaced knobs that specialize in rolling fast, shedding mud, and being decent even on the road.  It is a terrific knobby fatty for any 650B bike that can fit it.

Actual dimensions
Radius: 350mm
Width: 49.89mm to 51.13mm (casing, knob)
Weight: 535 grams