For the most part, it's a 306 (XS2) engine.
Basically, the 70-73 engines share the '256' conrods, pistons/rings, cam and camchain.
The original '256' crank has no starter gearcuts, which were incorporated in the '306' crank, and supercedes (replaces) the '256' crank.
With the 306 (XS2) engine, many changes occurred, including the starter system (with reduced oil capacity), new left cover, new right cover and oilpump, new head/camcover, compression release, new lower 1st gear and matching kickstart gear, new 6-hole camchain tensioner parts, and cylinder block.
Pouring thru the 70-73 engine parts lists, including the separate '73 TX650 parts lists, I can find no '366' specific engine parts, it's mostly the same '306' parts, with a couple of sub-revision suffixes.
Sub-revisions occurred throughout the XS lifespan, often in mid-year. Significant ones were covered in Service and Parts bulletins, others were mysteriously slipped-in at various times, probably due to changes in vendor specifications or sourcing. You can find these in my albums:
http://www.xs650.com/forum/showthread.php?t=31022
The '366' numbers appear to apply to bodywork only...