The Nikon F Photomic FTn is the original Nikon F fitted with the final and best of its metering prisms, the Photomic FTn finder of September 1968 - what UK sellers listing a Nikon FTn body almost always mean (the cheaper hinged-back Nikkormat FTn is a different camera).
The FTn finder brought 60/40 centre-weighted TTL metering and the famous Nikon-shuffle aperture indexing to the all-mechanical F: titanium curtains to 1/1000s, 100% finder coverage, interchangeable finders and screens, native pre-AI mount.
It is the definitive configuration of the camera that made the SLR the professional standard - the body of Vietnam-era photojournalism - and the most affordable way into an original F today.
Used buying: confirm identity first (removable finder, back-and-baseplate removal, not a hinged back); expect the meter to be inaccurate or dead (mercury-cell calibration) and price accordingly; check prism foam decay causing desilvering lines, verify speeds by ear at both extremes, inspect titanium curtains for creases.