Look like home-made salute gun. Without pivoting trunnions you can't elevate it to hit anything. I don't think Pancho was that hard-up to drag something like that around.
I'm not too sure about that.
This gun clearly is a "homemade" piece, probably built in some of the railroad workshops where supposedly shells and other guns were also made. You can even see the lathe marks at the muzzle and the shape/style of this gun doesn't fit to any other gun I've ever before seen.
The other homemade guns that I know of were bigger rifled breechloaders but, there were also small, odd-looking guns called "dynamite guns". These "dynamite guns" are the reason which make me believe that this really could be a cannon used by Villa's troops. The dynamite guns were similarly constructed, a very simple shaped tube on a very small carriage without any elevation mechanism. Mounted in very similar way also, by a strap over the barrel, bolted to the carriage.
I'll see if I can find one of the photos showing those "dynamite guns", but the newspaper article about the raid would also be great thing to have.