To answer the question "were they high-end Manhattans" - Here's my observation. The basic F515 four door sedan model that we had did not have all the fancy interior trim our 1949 Frazer Manhattan had. The 1949/50 Frazer Manhattan had extra chrome trim on the dash, chrome interior window frames, chrome trim on the shift lever and signal light lever knobs (which were color coordinated to match the upholstery), wide chrome eschutcheons on the window cranks and a chrome trim bar on the steering wheel which said "Frazer Manhattan" on it. It also had a fancy, color-coordinated rear seat ashtray and other fancy trim. It was tutone in color and had some extra exterior chrome as well.
The 1951 Frazer four door sedan we had and all others I've seen, had none of that and were not available in tutones, which would lead me to believe they were based on the 1949/50 Frazer standard model. Again, this is only my observation and not based on any KF factory documentation.
For 1949/50, the Frazer Manhattan referred to trim, a dual throat carburettor, higher compression head and most were tutone. For the 1951 Frazer, the Manhattan referred to a four door hard top or convertible, and also had extra chrome trim and most had a Hydramatic transmission which was not available on the 1949/50 cars.
Hope this helps. I'm sure the historian will shed some more light.