Hi q-x
I am following this with interest because I have some Ricoh slrs and my favorite slr camera is the newer, small and simple KR-5 Super 2 which weighs about 440 gram.
These K mount cameras meter with the lens at fully open aperuture, and the metering reads the aperture setting which is conveyed to the circuit by the lever coupling on the right side of the mount opening ( when viewed from the front). The body lever is on a ring spring loaded to the upper acw position and it engages with the lens lever when the lens is rotated onto the mount.
The metering is center weighted, so to do a simple lens by lens metering test it is necessary to use lenses with same focal length on the same image.
Reading this thread I tested 3 cameras set at iso 400 and 3 lenses
Lenses
Ricoh -M 1:2 50mm at f/5.6
Pentax -A 1:2 50mm in M mode at f/5.6
Magnon Zoom 1:3.5 35~75mm at f/5.6 and 50mm

Cameras
Ricoh XR-2 with Cadmium Sulphide ldr sensors
Ricoh KR-5 Super2 metering sensors are not stated in manual
Pentax MX with Gallium Photo Diode sensors

I also used a Pentax 3/21 spotmeter (Cds) which metered the center part of the image (a larger lamp shade) at iso 400 f/5.6 as 1/125th
Testing all lenses on all cameras, the 2 Ricohs were identical, each predicting 1/50th on all lenses
The Pentax MX consistently predicted one stop lower, at 1/25th on all lenses.
I did abbreviated test in daylight on snow to show the same, the MX is one stop slower when the ricohs predict f/5.6 and 1000th.

All cameras produce good negatives so I am not concerned about the difference, it could be calibration, or in the different center weighting curves between the Pentax and the Ricoh.