Fstop, have you read Tom Abrahammson's review of the Zeiss Ikon system? He is the only one I know of who has specifically addressed the quality of the finders. If you already have a 21 finder & are happy with it, I don't see why you would add the expense of a new finder no matter how good it is- especially at the price that Zeiss is charging for these. If you haven't seen it, you can find it on the link below. His discussion of the finders is at the bottom of the page, toward the end of the article.
Zeiss 35/2 Biogon, Zeiss 50/1.5 C-Sonnar, Rollei 40/2.8 Sonnar, Voigtlander 28/3.5 Skopar, Voigtlander 75/2.5 Heliar
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