I assume you are referring to the A/M switch on the lenses - which work the same way on all the different Mamiya 645 bodies.
When A is visible/exposed, the aperture will remain wide open until the instant of exposure, automatically stop down for the exposure, then automatically reset to wide open. That is how it permits open-aperture metering.
When M is visible/exposed, the aperture works manually - when you change the f/stop setting ring, the aperture will change to and remain at that f/stop. You can use the M setting to help visually evaluate depth of field at different apertures.
The A/M setting has an affect on any automatic exposure provided through the appropriate metering finders, but isn't actually referring to automatic exposure or manual exposure.
I looked at three different lenses from my collection of five, and they work this way. There is at least a possibility that some lens in the extensive range of lenses made by Mamiya over the decades for their 645 system implemented the A/M switch in a slightly different way, but I can't recall one.