Mamiya M645 AE meter overexposing 2-3 stops

My Mamiya 645 with AE meter needs the ISO reading to be set 2-3 stops high in order to provide correct exposures, i.e. I must have it set to 1600 or 3200 for 400-speed film in order for the indicated readings to resemble Sunny-16/external meter. I've googled a few times and haven't really come up with any ready explanations.

New Energizer A544 6v battery (non-silver oxide)
Clean contacts (no corrosion) in battery compartment and between finder and body
Lens barrel set to "A", so aperture not stopping down

Any ideas? Could the finder itself just be half dead? This has been the case since purchasing the camera a few years ago, finally got annoyed enough to ask.

ETA: Someone on another forum suggested to me that the focussing screen could be an issue but I believe I have a stock Mamiya screen (at least, the one that came on the camera when I bought it). 45-degree split screen surrounded by microprism, a Type-E based on this scan: https://ianbfoto.com/downloads/Mamiya 645/Mamiya_645_Focusing_Screens_Product_Sheet.pdf

Which exact model of Mamiya 645 do you have?
I ask, because that sheet references the N screens, which means, AFAIK, , the screens that came out with the 645 Super.
If your camera happens to be a 645 Super or 645 Pro, then the interchangeable film back and its connection come into play as well with the AE metering.

Apologies for not specifying; it's an M645 1000s.

After more searching, I believe it's the #5 screen from Mamiya, which I assume is native to the camera.

Start by verifying the reference voltages: VR, VK, VAD.

Ah, yeah, that's probably the best thing to do but I lack a volt meter and it's more effort than it's worth. Just an odd issue that I expected to be discussed at length over the years and was surprised I haven't seen anything...

That is a pretty nice camera. If you can't calibrate it yourself, I'd think it would be worthile to have someone look at it. For example forum contributor Mamiya_Repair.

It's a silly suggestion perhaps, but can you verify that the aperture linkage between the lens and the metering prism is seated correctly? With this camera, it's fairly easy to mount the lens incorrectly so that the aperture linkage is off by a few stops. There's a pin that needs to fall into a notch at the top of the lens: