If memory serves, the 105mm howitzer they use is an M102 light towed, demounted from its usual home.
Pretty much. As when the USAAF took the 75mm main gun from the M24 light tank of WWII and mounted it in the B25 [H-model, IIRC] twin-engine bomber, the Air Corps/Force concocts its own designations for things, GAU-this and GAU-that. [Gun, Aircraft Unit. It pays to have someone in the household who speaks USAF]