The user manual states (in manual mode an other than A setting):
"When a lens other than an F or FA lens is used, no approximate aperture indication will appear in the viewfinder."
It also says:
"When using a Pentax A f/1.2 lens with the aperture ring set other than the A position, the center weighted metering mode will be set instead of the multi-segment metering mode. As the exposure will come out 1 stop over, set the lens aperture ring to [ A ], or adjust the exposure deliberately 1 stop under."
-pg 61 of manual - available here:
Pentax Manuals
DA and D-AF lenses should show the approximate aperture but the manual was written before these lenses existed