Soviet agents were able to secure plans about Christie's tank through underground links to an American Army officer. Though the Soviet Union had no diplomatic relations with the US at the time, and was barred from obtaining military equipment or weapons, Soviet OGPU agents at the trade front organization AMTORG managed to secure plans and specifications for the Christie M1928 tank chassis in 1930 using a series of deceptions. On April 28, 1930 Christie's company, the U.S. Wheel Track Layer Corporation, agreed to sell Amtorg two M1931 Christie-designed tanks at a total cost of $60,000 US, with the tanks to be delivered not later than four months from date of signing, together with spare parts to the purchased tanks for the sum of $4,000. Rights were also transferred to the production, sale and use of tanks inside the borders of the U.S.S.R. for a period of ten years. The two Christie tanks, falsely documented as agricultural farm tractors, were sold without prior approval of the U.S. Army or Department of State, and were shipped without turrets to the Soviet Union. The Soviets later improved upon the basic Christie tank design, adopting its sloping front armor for its BT tank series of infantry tanks. The BT itself was further refined into the famous Soviet T-34 tank of WWII, retaining the sloping front armor design, now adopted for side armor as well.