Are you referring to the “Meter Control Button” on the lower left side (as you face it) of the lens mount?

See page 40 here.

Note: with the “Shooting Mode Selector” set to “Manual” the meter is in center-weighted mode regardless of the position of the Meter Control Button.

The second part of the manual is here

The manual ought to answer most of your questions regarding the use of the FA.

I've no idea why the MCB would be missing. Tampering by a previous owner sounds possible.

In normal operation you'd push in the MCB to override “Automatic Multi-Pattern Metering.” That sets the metering to center-weighted in any of the auto exposure modes: Program, Aperture Priority, Shutter Priority. Otherwise the auto modes use AMP metering.