I have searched these pages and looked at the viewfinder mask chart but I wasn't able to interpret what mask I really need for a 135mm lens on a 4x5 Crown (top rangefinder) with an RH-10 back (6x7 I think). Any help would be appreciated.

Charles,

According to the Mask Selection Chart in the Graflex Rapid-Vance RH/20 Roll Film Holder Guidebook (Form No. 1804/865), for a 2 1/4" x 2 3/4" "picture size" (i.e., negative) with a 127-138 mm focal length lens you would need Mask Catalog Number 9105-10. That translates to a number 10 mask. (The fact that your holder is an RH10 rather than an RH20 shouldn't matter for this application.) Hope this helps.