To clarify for OP... How the ISO dial links to camera behavior seems to be the point of confusion...
The Pentax ME Super doesn't have an automatic aperture. The f/stop you click is the one you get.
Some cameras override the aperture in program and shutter priority mode (OM-Program comes to mind). Pentax ME Super doesn't mess with your f/stop once you set it.
The camera has an electronic shutter. When using Auto - Automatic mode, shutter speed varies automatically and the speed will change by 1/3 stop if you make 1/3 stop change to the ISO dial. (But don't do that for filters since it reads the light through the lens).
When using M - Manual mode, you get the shutter speed displayed in viewfinder - the shutter does not change 1/3 stop when you change the ISO dial.
Going to the original plan: Setting meter ISO "B" to 125 is a good idea. Leaving the camera at 160 is a good idea.