Probably a crew-served weapon of some sort, or something mundane, like rations. There are quite a few other things adapted to airborne service, including a mini-bulldozer (though not used on D-Day, they were handy for clearing flight strips in the CBI, etc. It could also be part of a medical aid station, as those were needed everywhere.
A trip to the 82nd Abn. Div. Museum on Ft. Bragg is really worthwhile, but it's been over 40 years since I've been there.
Now that I think harder about it, it might be a 105mm or 75mm pack howitzer, as that's how those were delivered in some instances (otherwise, they arrived by glider). It's been a long time since I've even thought about it.