GWR coach roof ventilators
courtesy Pete Speller
These photos, courtesy of Pete Speller of GWS Didcot, show dimensions for the "Ash's Patent Carriage Ventilator" (which we colloquially call shell vents), as fitted to GWR coaches. The vanes are held by a rivet each side, and this particular vent has received a bash or the rivet has become a bit loose, which accounts for the wonky appearance in the second picture.
The vents are mounted perpendicular to the profile of the roof. These two roof profiles, of an E127 and a K41 respectively, show their lateral spacing, which for most GWR coaches was 1'0½" from the vehicle centreline. (There were some exceptions to this dimension.)