The F model is a mid body with a nice performance curve - basically an unclean J model, which is as good as it gets. Some F models have the speed mods and look like J’s, but people also like to price them as J’s.
Not to be pedantic but there’s other differences other than some speed mods.
The 1st year the J was introduced (77) was a transition year, it retained a lot of the Fs design features, except in addition to speed mods the engine intake design was modified, this resulted in the ram air mechanism becoming superfluous and it was eventually dropped.
In 78 the gear was modified, which up the speed you can drop the gear, the new aerodynamics made it harder to slow down, and speed brakes weren’t available yet. They added an annunciator to cleanup the panel a bit and add additional fuel/electric warnings.
They beefed up the support structure that allowed them to up the MGW around 87, which they needed as it was getting heavier because of other improvements like fully adjustable seats, winglets, etc. Even with the higher MGW the useful load isn’t that much different than early models. A 1000lbs is considered quite good for any J, but Fs can be 1050.
That’s just the highlights.
The F went through changes as well through the years, don’t assume they’re all going to be exactly the same.
BTW, all Js have a serial number of the form: 24-####. I have seen Fs and even Es with a few J speed mods that listed themselves as Js in a effort to get a premium price or some claiming to be better than a J…buyer beware.
