Focusing Screen Query ( MX )

Lost screw from Pentax MX that held the winder plastic on.
Bought 2nd hand broken MX from shop to replace screw.
Noticed when comparing 20mm Cosina & 20mm Pentax lenses that one viewfinder was brighter than the other. Swapped lenses over and the same camera was still darker so it wasn't the lenses but two identical ? MX cameras, puzzling.
Noted that the brighter one has an SC1 screen the darker one has an SC21 screen that was a replacement about 8 years ago.
Now according to the Dimitrov Pentax site the 21 should just be a later more up to date version of the 1 so it's darker ? The 21 is also I believe standard issue on the LX.
So the old screen is brighter than the new one ?
Comments please.

The newer screens were supposed to be brighter and apprently affected metering ont he MX, so might be worth checking on your two bodies.

The metering measures the brightness of the ground glass screen, so a brighter screen will affect things.

Pentaprisms can deteriorate with age, so maybe one viewfinder is brighter because it's in better condition?

or indeed the mirror could be losing its silvering?

Hi All, I swapped the screens over and it does seem to be that the SC1 is brigter in both bodies. I suspected the pentaprism initially as my old MX has seen a lot of use and the other one was a bit newer due to having a higher serial number.I'd forgotten about the mirror so that's possible I'll have a look.