As I understand the toothed belt (synchronous drive) design has changed with time as a lot of things. The classic "Gilmer" or trapezoidal tooth, which works fine for moderate loads, evolved to a "curvilinear" tooth design which was called "high torque drive" or HTD belts. A rounded tooth. That then had minor design changes to become the Modified curvilinear tooth design with smoothness and excellent torque delivery and belts designed for it are, ( according to Gates anyway), the best high strength belts (Polychain GT Carbon) belts available . So far of course. These are the GT2 belts and pulleys. . They are pretty stiff, at least when new. Best I can tell, just looking at the teeth there is not much difference but you are not supposed to mix up GT2 and regular HTD belts and pulleys though they may physically fit. All three of these belt types- Trapezoidal, Curvilinear, and Modified Curvilinear have been used for cam drives by the OEM's.
Jesel cam drives use a custom width/length HTD belt, for instance.