Ash
Even when it is cleaned or clean the Contax will be dim compared with a M2. I suggest your first experiment is to compare your M2 with another M kinown to be in good condition. That is even if the poor M2 is beaten up so its finder is damaged it will be ok in lots of dim light conditions that a Contax finder will look like the black hole of Calcutta.
A degraded M2 would only be a problem for a photog who primarily photos at night, or perhaps contra jour.
So unless you can see muck in the Contax finder it may not be effective doing anything, the clue is the diameter of the eye piece, in each camera, If you want to be annoyed then you need to look through the Zeiss 'M'.
You will note that the M2 is the cheap productionised finder that the rest of the M series adopted, after the M3.
The M3 finder and the fast bayonet mount was the statement that Leitz made to retain their position, as the 35mm cam for prof, the Nikon mount was not competative until the Nikon F, no one competed with Leitz rngfdrs until CV and laterly Zeiss.
HCB used a M.
Noel