I just read the captions on the photos on pgs 10 & 11 for the first time. I see that the primary weapon is the same in both - a 75mm gun. And you could say it is a mobile artillery piece in both cases. The main difference is the source of mobility. One is a major polluter while the other has just a small carbon footprint. My guess is that after they submitted a Enviromental Impact Report on the M-1 they were forced by the EPA to scale back. The result of the redesign is what we see on page 11.
Good point. Though one is a howitzer and the other is a cannon, they are pretty much the same weapon. It looks like the howitzer is definitely more environmentally friendly. I would worry though that PETA might get wind of how we are overworking those poor horse by making them tow that heavy weaponry.