That will be the F33 ‘N over inverted N’, you have it in your Freeman re-print Richard and it is in the Gouby update too. Other than the photo of a worn second that Larry has previously shown, I haven’t seen another despite several years passing and it does seem to remain very rare indeed. Still in my collection of course.
As regards the ‘rarest’ coin in my penny collection I have several that are still in the ‘single figures known’ category from the Rarest Pennies database, with the above F33 and an F15 BRTTT perhaps the rarest. In my hammered collection I do have two of the four known 1558 dated mm lis Elizabeth Ist pennies, one being the Comber example.
Jerry