Gerry, when you remove one of the incident light attachments (flat disk or hemisphere), and replace it with the reflected light disk (with the hole in it), the Autometer IVf had a metal pin in the head which detected which type of attachment which was in place. Defection of the pin would alter the sensitivity of the meter...if you removed the attachments and actuated the pin while taking a reading, you could see the result change on the displayed suggestion for exposure. The Autometer IVf might have issues, with the pin deflection not working properly at times, so if you actuated the pin by hand a few times, that would usually restore correct pin function. Sometimes such manual actuation was not sufficient, and the meter needed to be sent back to Minolta for service.
The Autometer Vf does not have this metal pin in the head, but instead has what looks like a plastic attachment rotational-stopping feature at about 5 o'clock position. If you actuate this (push it down, flush with the surrounding surface) it should change the reading about 5EV when working correctly.
The reflected light attachment as an angle of acceptance which is equivalent to using a normal lens with a camera...about 45 degrees.
I have the 10 degree spot meter attachment (purchased when I had an Autometer IVf) and it is quite accurate.