Something I've noticed as a feature exclusively common to reverse F, is that on otherwise fine or even good fine specimens, Britannia seems to have worn away far more than other areas of the coin, which leads to that characteristic flat look, extending downward into the centre of the shield, rather than the shield being individually affected.
My own F32, which I'd otherwise describe as GF, is a prime example. Just wondered if anybody had any thoughts as to why. It does seem to be a pattern of wear unique to reverse F.