One thing that must be taken into account is that the F164a was not discovered until after the recall of all the pre decimal coinage , and that means that none were syphoned out of the vast numbers of the 1908s still in circulation back in 1967. This disproportionate survival rate of known scarce and rare types can be seen with the 1912 and 1912H , as when I was young in the 1960s the 1912s were really common, but the 1912H was scarce, but if you look on e-bay today there are far more 12Hs than 12s due I think to the hording of 12H and the 12s being withdrawn and melted down. I would personally think that the F164a is as rare as R15 ie 101 to 250, this judged by the numbers turning up and the prices they attain. As you say more are coming to light all the time, and its down to the internet , and the high quality of the pictures now available to us making them easer to spot. just my opinion but I think that they are just a little rarer than the open 3 ,