Hi David! ;-) quoting from http://medfmt.8k.com/mf/mamiyatlrlens.html

Roland wrote
I've taken apart both viewing and taking lenses for cleaning and both
the taking and viewing lenses look identical to each other and are
constructed the same. This includes taking out individual lens elements
for cleaning. I've swapped them round and noticed no difference at all.
They would seem to be identical twins. So next time you see one for sale
on ebay with a slightly spoiled taking lens then help yourself to a
bargain.
endquote

these posters suggest they are the same for these mamiya tlr lenses...

In many other TLRs, of course, the viewing lens is faster so as to provide
narrowed DOF and sharper focusing, or less complex as it is not being used
to make the photo, just an image to focus (fewer elements etc.).

if you do swap lenses, be sure to recollimate the new setup, as shims will
likely need to be set to make them match up again...

hope this helps bobm
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