Once again Louie Turner has demonstrated a remarkable ability to inject even more confusion into an already confusing situation. There's an old saying, "You can't teach what you don't know." (If that upsets you, Louie, have your lawyer contact me.)
GO #33 WD 1945 as amended = War Department General Orders Number 33 of 1945 (the official authority for the campaign credits listed)
As the designation implies, this was an antiaircraft unit, the stated or officially-prescribed mission of which was to defend ground elements or installations against attacks by enemy aircraft (some who served with these units - especially those attached to infantry and armored divisions - claimed "AAA" actually stood for "anti-anything anytime, anywhere"). Battery D was one of the battalion's firing batteries. The battalion was equipped with single-, twin- or multi-mounted .50-caliber machineguns and/or 37-, 40-, 90- or 120-millimeter single-, twin- or multi-mount automatic antiaircraft guns, towed or self-propelled. Type of armament varied from one battalion to another.
All the above - as well as the psycho-babble of Turner's replies - may well be moot. That the above is indicated on his discharge means only that it's the unit he was assigned to at the time he was processed out of service. May or may not be the unit he served with overseas. It's probably safe to assume he was an antiaircraft gunner and provided air strip security - among other duties - but there were well over seven hundred such battalions active during the war. He may well have merely rotated with the 951st, having served with another battalion - if, in fact, he was even assigned to an antiaircraft battalion. The only way to determine for certain is to access records.
142d Infantry was an element of 36th Infantry Division and served in Europe. There may have been some sort of association - personal or organizational - with the regiment in the U. S. but obviously none overseas. 161st Infantry was an element of 25th Infantry Division. Antiaircraft battalions were routinely attached to ground units in the European Theater but no so in the Pacific - although they very often provided supporting fire. Again, the only way to determine an association is to access records.