I could very well be mistaken, but I'm pretty sure that is a 75mm pack howitzer. My first guess is M1A1 with M8 carriage, but that is based on a very limited amount of knowledge, namely, a quick Google search.
TM 9-319 is the manual, and apparently is available at the following:
http://www.ahco.army.mil/site/index.jsp
use "TM 9-319" in the "title" search block, and scroll through until you find it.
I just discovered a totally new way to waste time. That website looks like it has some absolutely amazing stuff on it, not the least of which looks like every weapon TM going back many years.