"Mach Board"?? Wouldn't that be board exams at the speed of sound? On the Concord perhaps! 😀
I know what you mean, though. I think most programs have upper level residents take the ABIM in-training exam. I've never taken it, but from my understanding, it's not exactly like the board exam, but your scoring correlates with your scoring on the board. Check with your PD.
(btw -- I think you meant "Mock boards" -- as in fake.)