Back during the '70's "energy crisis", AMT tried to the two part tires as the used a different plastic that the one piece vinyl tires. Some of the car kits also had plastic tires instead of vinyl. (FYI- pre-energy crisis vinyl tires are the ones that melt the styrene wheels if you didn't pant the contact surface). If you have any of the old kits, you'll find that the plastic on some was "milky" while others would have the white plastic turn turn tan or yellow. Both the AMT and Revell plastics were also more brittle then they are today. Those were interesting times for modelers.
A.J.