Thanks all. So I guess the finder is square because of the revolving back. I guess if the points of the triangles are the limits of the image area, then I'm hoping it's the limits of the short dimension. If it's the limits of the long dimension, then the short dimension is inside that and not marked. I'll have to see when the retaining ring arrives and I can mount the lens to the lens board. The Watson viewfinder is nice; all shiny chrome and has a spring-loaded mechanism to lift the rear up a few mm when you release a catch, which gives a single fixed parallax compensation. Not continuously adjustable like the Graflex one, but it looks cool. It's also mounted closer to the centre of the camera top probably because there is no horizontal parallax compensation.
Another thing about finders I only learned while researching this relates to the wire frame sports finders on Pacemaker Graphics. The design changed in the mid fifties. My newly-delivered Crown Graphic dates from about 1957 and has a flexible coiled wire frame in front with no parallax adjustment, but has the rear adjustable height peepsight. All the parallax adjustment is done with the rear peepsight. (It's also an unusual Crown because it's from 1957 but has a side Kalart rangefinder on it.) My first graphic is a 1949 Pacemaker Speed and has parallax adjustment on the front (rigid wire) frame finder, as well as different pull-up heights on the rear peep site. I always thought you were supposed to adjust BOTH for the distance, but it turns out the rear peepsight on mine must be from a newer Graphic. The original equipment on the earlier Pacemakers had a fixed height of the rear peepsight and all the parallax adjustment was supposed to be done from the front wire frame.
