Brand new these pots had shiny chrome interiors in the lower pot. Because many owners were lazy and didn’t wash out the pots between uses the coffee oils built up and stained the inside.
An easy way to clean these pots is to run a full pot of water with a tablespoon or two of powdered dishwasher detergent (or 1 DW detergent pod) through and brewing cycle. Then unplug the pot and let the solution sit in the pot for 30 to 45 mins. Then pour out the dirty water, and it will be dirty. Now run a brewing cycle with plain water only, pour it out, rinse and dry the pot. it should sparkle like new.
The dishwasher detergent is the equivalent of old fashioned Dip-It coffeemaker cleaner which is no longer the same formulation as the old days of folks using electric percolators and vacuum pots like you C50.
HTH,
Eddie