Inserted into the accessory shoe atop the prism, it looked very much like a large rangefinder zoom finder. At least one 7 fps model was made or converted to use the 250 back. This was apparently Nikon's reply to Canon's faster 9 fps pellicle F1 High Speed, also made for the 1972 Olympics. Vs 9 fps Canon and SLR viewing. Yep, this was one of the first times Canon could brag of beating Nikon at anything, see Canon High Speed .
More research needs to be done on the production.
If you have a F High Speed, please email me the serial number. 7 made for the 1976 Montreal Olympics, all converted from the 7 fps model. 4 of the 7 have the Canon Pellix mirror. The White Nikons became his trademark on international photographic trips for many years . Amazing, just amazing.