Infantry divisions had, as a part of their standard table of organization, three infantry regiments, Roger. (Incidentally, the phrase is "order of battle" rather than "order of combat".) These regiments were a "part of", or an "organic" element of the division. Some regiments were formed, and performed service, without being an organic element of a specific division. They were refered to as "non-divisional", or "separate". They would be attached to larger units (such as divisions) to add support, where and when neeeded. It appears the 159th Infantry was one such regiment.
Stanton's OoB states, in part, the 159th Infantry, "arrived in France 18 Mar 45 where attached to the 106th Infantry Division throughout European service and entered Germany on 25 Apr 45; returned to New York 4 Nov 45 and inactivated at Camp Shanks NY on the same date".