What main jets are these??? Holley 600

Because of the tolerances of the holes. So in the standard sizes, a #66 = approximately 0.066" diameter. Maybe the hole is .0658"-.0662" or something? But they never checked the actual flow. The "close tolerance" jets like your #661 is supposed to flow exactly what a #66 or 0.0660" diameter jet is supposed to flow. They were used to meet emissions regulations.

EDIT: I may not be 100% correct here. But you get the idea. It really doesn't matter if you use a 66, 661, or 662 in a pre-emisions engine.