ON 32MM frame rails, measurments are as follows ....... Internatoinal = 3 5/8" BETWEEN the rails OR 5" center line to center line of the tube . American = 2 3/8" BETWEEN the rails OR 3 1/2" center line to center line of the tube. There are a few MFG that use there own mount , like CRG. I beleive..... That have a odd spacing.
And fwiw, the stamped steel butterflies suck. See if you can find a good billet aluminum one, particularly if it has a full "saddle" that wraps around the tubes. Much less likely to slide or distort shape and loosen up.