I'm no lens guru, but my LVM tells me this:
Universal Lenses for Portraits and Groups, in Studio, Interiors, Views etc. These were made in a back focus of the following: 7.5in (2.0in), 10.75in (2.25in), 13.5in (2.75in), 16.5in (3.25in), 20in (4.0in), 24in (5.0in), 30in (6.0in) where the diameter in inches is in parenthesis, and corresponding to about an f6.0*, possibly a Portrait RR, since they were "flat field". They covered 40-50° and 13.5in was suggested for 10x8in. (*It is likely that internal baffles would actually limit the useful aperture to about f8."
But as usual one has to be careful with the LVM info, never take it for granted. HTH