Are we sure this is not a 105 "Recoilless rifle ' ? The mth Howetizers resemble 155s and not the 105'.. that would justify the longer barrel... remember rarely are these accessories actually perfect 1:48 scale. Probably closer to 1:43, 1:45...
When I was in the first Armored Infantry Div. our M60A1, 120 ton tanks, tracks, hung way over the rail cars. Especially when on exercises in Germany, when we moved equipment by rail from Graphinvere to KT on Reforger war games... So Ed, your tanks tracks hanging over are prototypical...