Sanders, my guess is that the Rittreck holders won't fit. I have a set of the Rittreck holders to match my Rittreck camera with "yatsugiri" adapter back; they're too long to fit my older American WP cameras (Eastman, Century, Gundlach Criterion). But since they're on the way, soon enough you'll be able to check for yourself.
A bit of trivia: the Rittreck yatsugiri holders actually aren't "Wista" holders - they were made by Tachihara, as was the yatsugiri adapter back for the Rittreck.
Oren, alas, the education comes after
the horse has bolted the stable. "Too
long" would seem to be remediable
but as you say, I will soon enough
see for myself. If the holder can be
made to fit otherwise, will it hold the
film the proper distance from the lens?
Well... that depends on your definition of "proper". How close do you need your ground glass register to be?
Allowing for typical sheet film thickness, none of my American holders precisely matches any of my American backs. The Rittreck holders show an even bigger mismatch with both the Rittreck back and my American backs. The best match, between some Hoffman holders and my Century and Gundlach backs, is within about 0.003"; my Eastman holders are off by about 0.010" from my Eastman back, the Rittreck holders are further off than that compared to both my Rittreck and American backs, and there are other combinations that are worse still.
I wouldn't place much weight on these exact numbers, nor would I run out looking for Hoffman holders to match with Century or Gundlach cameras, just on the basis of this one set of measurements. There can be variation in cameras and holders even within a brand. And film holders turn out to be devilishly non-rigid when you start attacking them with a micrometer, so there is almost certainly some slop in my numbers.
But the take-home is that in WP-land, if you must have very accurate ground glass register for your intended uses, there are three possibilities: either you get very, very lucky; or the mismatch is in a direction that can be fixed by shimming the glass; or you need to have a set of holders custom-built to match your camera.