Oh well -- it had to happen some time -- I'm back in the 'Contax' fold ( albeit the JAPANESE Kyocera version!) when my 'first' 35mm was a pre-war Contax III with un-coated 50mm f1.5 Sonnar bought in Berlin Germany when I was there in 1956 with the British Army , for £ 38-00 . Now I have been GIVEN a RTS II Quartz and a Yashica FR1 with various lenses, flash, cables, winders etc etc -- see my post in section '35mm Cameras and Accessories' -- I have tested BOTH cameras : they both had been 'banged'-- the Contax on a corner so frame counter starts at 12 and Yashica on pentaprism and frame counter stays on 'S' so I have to guess how many frames I've taken ! But the Yashica lenses 28, 50 and 70-210mm zoom are good and fungus-free as is the Contax 50mm f1.4 Planar -- I had to replace the mirror foam on both cameras as on use after so many years of non-use it all crumbled rotten -- the next foam to go will be the door hinge one. I notice the CONTAX metering is NOT accurate when a RED filter is used -- it under-exposes -- I did some tests relying on the expensive 'Real Time' exposure metering and the ones with RED filter were very THIN negs compared to a manual setting from a reading from my Weston Master V and allowing three stops extra for filter density. I had to put in a new 6v battery in Yashica as one inside was well gone -- luckily NO acid leakage ! I had to wipe all the lens cases and put on some ' Neatsfoot Oil' as they were all covered in dust as were the lenses but luckily no Fungus !
I will bore you with some results now in Group Photos -- please read under the photos to find out which is Contax and which is Yashica.
Contax RTS II Quartz by pentaxpete, on Flickr
Yashica gear by pentaxpete, on Flickr

Any idea why the red filter affects the metering accuracy? Do you have the same problem with an orange filter, but to a lesser degree?

I had a reply to the same query on the 'Amateur Photographer magazine' site by the famous Roger Hicks and he said it was the fault of the 'colour sensitivity' of the Contax metering cells being 'over-sensitive to RED' -- yes -- I tried with a YELLOW filter and exposures were better -- will try the Orange one next -- in the meantime 'Manual Settings' from a Weston Master V meter gives more accurate results than the very 'expensive Real Time Metering 'of the CONTAX !