Armored truck of the Mexican Constitutionalists

I am not fully conversant with the political scene in the Teens in Mexico, so I’m not clear on the circumstances which allowed a “rebel” group to have the resources to build an armored war truck on a two-ton Mack chassis. It’s a Herkimer Battle Jitney, except without the “non-lethal” part. According to The Horseless Age:

The truck was delivered to a Southern dealer, but the manufacturers had no idea of what purpose it was intended for. Armored flanged wheels are fitted, which permit of operating the truck on railroad tracks where any continued stretch of same remains, but by a quick adjustment the wheels can be changed to run over ordinary roads. Inside the truck body are carried ammunition, rifles, etc., and the fire of machine guns can be directed through small portholes in the walls.

Also, it drives through schools.