[RichardJ]

Every one I have dealt with was adjustable. Must be aftermarket/Chinese.
You could grind it down on a grinding wheel, shorter and with the same contour. If you ever replace the MC, you might have to weld the pin back up to meet that situation.
What a pain.
A permanent flange type gasket spacer between the MC and booster would work. Aluminum sheet would probably be best. Soda/beer cans make good shim material and can be cut with scissors.