Why triangle patch is not visible when view through viewfinder

where could I buy a new one beam splitter ,because the coating of beam splitter does not exist and when view through viewfinder the triangle from the rangefinder is not longer visible , and i try using a piece of blank 35mm color negative that has been processed which I put behind the beam splitter, and followed these procedures the triangle of beam splitter is visible , but appears as double triangle, probable first triangle is a reflection of 35mm colour negative film procesed , and the second triangle is reflection of beamsplitter glass.What do you think.Thanks

This is such an old camera parts will be difficult to find. Maybe you can find a second non-working camera to steal the part from to fix yours.

Thank you very much