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The least expensive and most practical solution to this request is to find a Kiev 88 prism in good condition. (probably on Ebay). They are well built,fit perfectly and are a solid performer optically. Can get from $50 to 80. I bought mine for $65 dollars and find it more than adequate for my needs on both my Hassy bodies.
if you live in USA or through ebay from Ukraine directly if you are from Europe.
I have a spare one left, though, as I moved to Rollei. Please note, Prisms add some weight to the camera. Calibration is easy. I would NEVER buy one w/o TTL.
--Rainer from Zurich
I have the chance to buy a used PM5 prism viewfinder for my 500 C/M. Does anyone know whether the viewfinder enlarges the image as does the loupe in the normal viewfinder? This would be a very handy feature...