A thread gauge can be a useful thing to own...
Back in the Rapid Rectilinear days there were no standards. Shutters were made in a bewildering number of sizes and threads - they were made to fit the lenses rather than opposite. Don't expect a lens from one maker to fit into a shutter for a lens from another maker!
If you have a couple of other lenses, you can estimate the focal length of the unknow one by using the technique described in this article: http://www.apug.org/forums/article.php?a=146 .
BTW, a Rapid Rectilinear almost certainly has "imperial" threads. If it were metric it would be an Aplanat (same lens, different nationality).