Two separate companies made each mold. The Darts were made by Highway 61 and the Demons were made by ERTL and now Autoworld since they took over Ertl's molds.
Greenlight now has control of the Highway 61 molds and are releasing some new cars using them. So far I've heard there are quite a few quality issues with these releases. Acme (formerly GMP) are also using the Highway 61 molds to much better effect.
