Mamiya Press 65 lens with vignetting

I have a 65 lens (early version) for my Mamiya Press. It gives a good performance other than suffering from unpleasant vignetting at most apertures. Having read various reviews/opinions no-one has ever raised this problem. I wonder, therefore, whether during its life -and it has certainly had some- the F & R element groups have been unscrewed from the shutter and some shims dropped out and not replaced. I don't know whether there should be any or not; its just a thought.
Has anyone any ideas why this particular lens should perform so poorly.
Thanks for your interest in looking

Shims should affect flatness of field and would probably not have an effect on vignetting.

Can you post a picture showing an example. Also, indicate which aperture you used.

I have the Horseman 65mm lens, also for 6x9 press, and it even allows about 20mm of movement.

ic-racer thanks for your interest

An example is attached and should show up the vignetting quite well. It was set at either f8 - 11 and I have deiberately left my shadow in the frame to show where the sun was (ie not behind the barn!) The sky was brilliant clear blue - I mention that as it is a bit of a rarity here in the UK!
I also have a 65 for my Horseman 970 - that lens is so much better than this particular Mamiya lens but the whole Horseman kit is getting too heavy for me to drag around in my advancing years.
Any help with the problem (with the lens) would be appreciated.

I'm going to say "Normal for that lens" but what we need is someone else with that lens to show an example.

That's pretty normal falloff of illumination for wideangle lenses of that era for that format. I wouldn't call it "vignetting," which more strictly refers to total occlusion of the image, for example, because the lens shade is too long, or because the lens doesn't adequately cover the format.

The in-camera solution is to use a center filter, if one is available. Heliopan makes some generic center filters, or you might try one that is recommended for another lens. While lens manufacturers often recommend a specific center filter for a specific lens, they're not as "dedicated" as the manufacturers would seem to apply, and they usually don't correct 100% of falloff. If they did, they would usually require more than two stops of exposure compensation, which is more than most users will tolerate, so they are usually a compromise between total correction and what is aesthetically acceptable, and a little falloff is often aesthetically pleasing anyway.

You can also correct in the darkroom by dodging the center of the print, as long as the original exposure was sufficient to have detail in the corners. Overexposing by a half to one full stop should be enough to split the difference between the center and the corners with negative film.