Steve, E.F. is what we now call focal length and it has the same meaning. It is the distance from the lens rear nodal point to the film plane when the lens is focused at infinity. Extension in general has little to do with that, depends on the lens' mechanical design. For many lenses the rear node is close to the diaphragm, i.e., close to the back of the shutter, so many of us have fallen into thinking that extension when the lens is focused at infinity is focal length. Not so.
Thank you for clarifying Dan. I had a vague memory that the determinant of focal length was nodal point to film plane but was unable to remember specifics. In this instance my test measurements have been taken from the plane in which the lens board was sitting when carrying a 180mm Fujinon. This gave me a distance of 180mm to the film plane when focused on infinity. However the Fujinon is not a RR design lens so the correct measuring point for the Versar is likely slightly different. However I have rigged up an aperture scale which I have applied to the Alphax shutter which should be close enough for practical purposes in exposing black and white film. This is based on my measured 9 inch focal length and measured iris diameters.
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