OK it's the T67 with the 75mm gun, The pic is taken at the APG in 1942 and only one vehicle was made. I don't have any more info on it
Ok I'll put you out of your misery Sam It's Swiss, a 75mm tank destroyer called the Nahkampfkanone II "Gustav", 4 in service 1942 until soon after the war. The chassis was a one off design, although I believe influenced by Czech designs. It was part of a weapons program designed by the Swiss to convince Germany it wasn't worth invading them, although by the time it entered service this was pretty unlikely anyway. AFAIK it was replaced completely by the G13 which was the Swiss designation of the Hetzer Jpz 38(t).