Is that a Jeep-mounted 105?
It's a Jeep-mounted M40A1 106mm recoiless rifle. That's pretty close to a 105mm, though the designation really came about as a means of assuring that 105mm recoiless ammunition from a previous weapons system was not used in the M40A1.
I'm after a right-side drive Land Rover, as used by the Australians in Vietnam, as a transporter for the thing- at 461 pounds, it's not exactly suitable for concealed carry....