Payton Products, Inc. Little Green Army Men - Pt 77 Sets: Miner Ind. Playset No. 1753 Army Action Squadron Task Force Set (my set #3) - Pt 5 Jeep and Howitzer

The box art for Payton's #2000 set, refers to this piece as an 'anti-aircraft' gun, but you'd be hard pressed to shoot down any aircraft with one of these.

Ed

The Miner Industries box top is a bit closer and refers to a 'mounted cannon' being included.