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
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
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.
1 stop off would be enough to ruin slide film, even with proper bracketing in +1/2 -1/2 manner.
KR5 Super 2 looks like a great camera, similar to my Carena in many regards Thumbs up!
I've used flat, featureless surfaces: wall, cloth, table, lcd, ground, with daylight, fluorescent and tungsten light where possible. Metering pattern has little to do in such cases, but I always remember to set focus to infinity, I think focusing from 0,25m/10in would result in measurable brightness drop, this is how close my 28mm lens would go. Regarding pattern, there are some cameras that are compensating for backlit subjects, and there are cameras compensating for light coming through the viewfinder, so I was trying not to let too much light in. Focal length - I'd say there may be a tiny difference in how bright image is formed with advanced viewfinders (fresnel lenses etc.), but it's only a speculation, and the difference can't be that big.
Initially I've noticed strange behavior of both bodies: one was acting strange in low light, while other was more lens-dependent. One gave greater variance, than the other. First issue is simple to explain - Carena doesn't work in low light, I have to live with it. Second issue was a bit harder, related to the measurement itself - Carena gives green light for way more, than 1/2 fstop range, while OK for me in Ricoh was within 1/3 fstop range. I'm quite happy to pin-point such issues and their outcomes... So I've reduced down amount of tested factors, getting rid of Carena body from the equation. I've eliminated battery issues with Ricoh and I've been left only with three lenses, rather than four, as the aperture tongue in my Revuenon, being off by 2.2mm, makes any testing with this lens pointless.
My only conclusion is 0.6mm of deviation within aperture transfer mechanism is enough to introduce considerable error. Smaller, than what Carena allows, but enough to confuse me with what Ricoh shows. so I'll have to trust my external light meter at least now and then, and I'll try to service Pentax-M lens myself, I hope it's doable with grinding stone in worst case, or screwdriver alone if things are designed to do it this way. For now, I'm unable to do anything more.