It's a British 6-pounder, but I think it's a Carrier, Universal, T16, Mark I, the US-built version of the original design which eliminated the track-warping steering system and incorporated a Ford Mercury engine, a longer hull with an extra road wheel, and some track and suspension changes.
Because of the more powerful engine and more robust suspension and rear differential, the Canadians put the T-16 into service in 1944 as a tractor for the 6-pounder antitank gun and as a 4.2-inch Mortar Carrier.
