A bit more information around those 1843DFF figures Mike.
My 5 year study of all ebay listings was between Sept 2007 and Sept 2012. There were 15 DFF's out of 130 listings of 1843's.
I think it likely that this is a fairly accurate reflection of true (minted) proportions, as I do not believe that many people were collecting Bramah variations at that time. For example, see Richard's comments where he got one by luck! The coin I have just sold you was the best example from those 15 on ebay during that period.
If I were to repeat the study now (which I will NOT be doing) then there is no way that you would see the same proportion of DFF's as there were approx. 10 to 15 years ago. More collectors have been looking for Bramah varieties since then, and most are now tucked away in collections. This is reflected by the fact that you have unsuccessfully searched high and low for one yourself for over a year.
The same could be said for the 1844 DFF's, seen on both the 'Plain Tail' 4's and the 'Tails Up' 4's.
I have done a breakdown of all the 1844's seen in the same 5 year period, and this is shown in the table below. Note that there were 60 examples of 1844 DFF's, highlighted in Yellow, so about 1 in 5 of 1844's had this 'feature' during my 5 year study. However, you have only seen 2 in the time you have been building your own collection of this series.
Hope the table makes sense, and is clear enough to read; the first split is based on being able to see the numeral 4's clearly enough to determine type.
Also note that the 'Tails Up' style of numeral 4 is clearly much more difficult than Plain 4, although there is a higher proportion of DFF's on 'Tails Up' numeral 4 type!!
It's not really an F of course!!