Ok, nobody has an answer. They were sledges attached under Sherman tanks, carrying 110 rounds of 75-mm ammunition, and apparently used on D-Day (according to Col. Merritt Hayes Bateman, The Sherbrooke Fusiliers Regiment, 27th Armoured Regiment -> From: http://gateway1.uvic.ca/spcoll/Mil/Oral.html ). The tanks go ashore, they drop their turtles at a rendezvous, they carry on fighting. If they need ammo, it is there at the rendezvous waiting. Ingenious.
The camo looks French, but I didn't think France really bothered much with tankettes. Though I could be very wrong. How about Belgian?