Interesting.
Never seen that before, Cliff. As Terry says though, there's probably many more varieties to be discovered yet, particularly among those first few hectic years of bronze production.
Reading Michael Gouby's book on the 1860 to 1869 period, it does appear that James Watt was not terribly efficient, especially at first, and I do wonder if some of the deviations from "norm", that we see, are due in part to working at high speed, cutting corners and an "anything goes" attitude in order to meet production targets. If that meant carelessly repaired dies, or a wonky 1 in the date, then that was a small price to pay as far as they were concerned.
With that said, he does seem to think that the scarcer varieties of 1861, which also contain flaws, as we know, were produced exclusively by the Royal MInt, as reverse dies were tested, against various obverses, prior to obverse 6 and reverse G being the accepted standard. That's not to say Gouby is definitively correct, however. After all this time, there has to be an element of speculation and inspired guesswork.
I think they should have stuck with reverse F. There's something oddly attractive about it, in my opinion.