I realize that the P4 is listed as exactly 90mm so my question is more whether it the correct one for this particular lens? Can't find a spec for the exact focal length of the Nikon SW 90mm f/8. If it's exactly 90mm I'm good to go, but I ask because I earlier incorrectly assumed that a 135mm cam was the right one for a Rodenstock/Caltar IIN 135mm f/5.6 (which isn't precisely 135mm though the contemporary Optars & Raptars are).

Anytime you manufacture something in mass, there is variation from one piece to another. Rarely is any lens it's stated focal length to a millimeter. In fact, Carl Zeiss started measuring their lenses in centimeters because 1/10 of a unit on either side was acceptable, but one whole unit (millimeter) was not.

From what Tim Holden wrote, Zeiss lenses tended to be longer than the stated length, Wolleys and Ektars on both sides of the target and Schneider on the short side.

Back when Graflex was up and running, you would have sent your lens to them, they would have measured the focal length to a 1/10mm and picked the right cam for that specific lens. On occasion they even tweaked a cam to make sure it worked, but to say that 'all Nikon SWs are exact, or short or long' would be inviting disappointment.

http://www.graflex.org/helpboard/viewtopic.php?t=4653&highlight=cam

http://www.graflex.org/helpboard/viewtopic.php?t=4647&highlight=