After looking at pictures of the Marine Kepi (which seems to 'bell-out' at the top), I'm concluding these troops are wearing the Army style of Kepi.  What looks like crossed weapons on their Mills Buckles could just be a reflection off the recessed "U.S." or National Guard Letters.  The device on their Caps could be crossed cannons.  The original caption "Artillerymen" is probably correct.   The artist may have taken liberties with trim colors when hand tinting the slide.  It's likely the rifles are trap-doors.