Webers only have 4 bolts, also in a rectangle pattern but different from the Mikuni pattern, hence the need for adapters.

Looking at the base of my old Mikuni I only see the 4 bolt holes, so perhaps any 5th bolt may depend on the model year or engine spec. I notice some adapters have a spur off the main rectangle, so that's prolly where the 5th bolt would be if present for your year/engine.