Dennis, post 16,
I have looked in our files and between the different porros made between 1945 and 1980 by different companies (e.g. Beck, Bush, Goerz, Kern, Möller, Voigtländer, Wöhler, Wollensak, Bausch and Lomb, Leitz, Zeiss, Krombach, Minolta, Pentax, Hertel & Reuss, Nickel,, Nitschke, Swarovski , Bleeker, Oude Delft-Delft Instruments, Tento, Komz, Konica) between 1945 and 1980 quite a few had transmissions from below 50% up to 80% and all values in between.
As Binastro in his post 18 already has explained a number of factors can be the cause of it.
Gijs van Ginkel