A standard US Army 155 howitzer from WWII until about 1980.
It's missing one of the gun shields.
Note the position of the jack on the front and the spade clipped to the side of the trails.
It was an excellent gun.
There was also another version of this gun, it was a 4.5 inch corps gun that went out of service at the end of WWII.
These were originally the US M1 howitzer, but they changed the name in the 1960's.