IPL > Illustrated Parts List
IL all they did on the part numbers was to add "SM" suffix to indicated that parts are from the Simplicity line [I think]. They are the same parts just a new part number so Briggs can track the correct part not get them confused with other OEM part numbers.
I think the problem is as follows.
Deep in the IPL on the deck lift it mentions a 44" deck so the longer belt is probably for that deck. It also mentions a 38" and a 50" deck. Apparently the proofreader didn't find all the errors or there was optional decks that are not listed.
And the belts are listed as "A" and "HA" belts meaning both are a 1/2" width. This maybe a case like the Murray deck belts I have seen where they have a different V profile. I once had a Murray that V profile was wrong on the replacement belt that had a deep V height. It slipped from the get go so I changed it to a Standard A belt of the correct length.