I know there's a chart in the tech info section, but there appears to be a misprint. And who sells these puppies, anyway?

You're correct, there is a misprint. Mask #9 in 4x5 is for 170-216mm and #10 is for 217-241mm.
These old dogs appear on Ebay from time to time. Also try the mainpage links to Mpex and Stephen Shuart.

Well, I finally found a #9 mask from Manfred Schmidt in Chicago and stuck it into the viewfinder. There's no way this thing comes close to what I see through the ground glass. The aperture in the mask measures 15mm x 11mm, exactly. Do I need to swap out the lens in the viewfinder as well? For what it's worth the camera originally came with a 135mm Optar.

No you shouldn't have to swap the lens out. You don't say if yours is a top RF or a Side RF with a removable optical view finder.

If your camera has a Top Range finder then it is what it is. If yours has a seperate, removable "tubular" finder, there is a slim chance it may be a wide angle finder. For 4x5 cameras there were two tubular removable finders, one for the 90-88mm set and another for the rest. All of the wide angle finders I've found are stamped in white ink "wide angle" onthe bottom of the finder.

If yours doesn't say "wide angle" then it's a normal finder and again, consider it an approximation. BTW which way is the view off? It is too wide or too narrow?

I haven't found any info that gives the true angle of view for either finder. If we knew that we could calculate what the opening should be for a given lens.

Does anybody out there have the "view finder repair" manual? '-)