Martin, the only possible theory I can promote for the apparently low number of very worn F14's is that most collectors, pre melt, were simply looking for LCW under foot, and at that time, were maybe unaware of the other F14 diagnostics. Given that the F14 was a very scarce variety to begin with, and that the LCW is an early casualty of wear, it's entirely plausible that worn examples of the F14 just ended up in the melt - the higher middish grade examples from circulation having been squirrelled away over the preceding decades - and the earlier back they were removed from continuing circulation, the more likely they were to be not so worn that the LCW had disappeared. So I'd bet most of the mid grade ones were collected before 1930.
I hope that makes logical sense, although it may not be right. Not simple to find reasons as there are so many variables, and so much completely unrecorded. So it's a bit of a guessing game. Hopefully others will also have ideas.