OK, so I actually did find out said truck was apparently a M109 shop van that had the box removed and a tank installed. Makes much more sense now.
As far as M49s, my first time ever driving a manual transmission was trying to roll-start a M49 with bad batteries. It was an interesting intro to a manual transmission!
I did get to ride along in another M49, which was very educational; a friend has all these aforementioned trucks and they gave me some verbal Deuce/manual transmission in general driver's ed in one. I loved it.