This is one of those catalogue errors (like 1854 half sovereigns?) that has persisted through the years - I guess it was originally catalogued either in error or assumption, and then subsequent cataloguers have just "borrowed" the information. Goodness knows how they arrived at rarities and values ("based on realised prices at auction" or similar when a specimen had never actually been recorded!!). It was in the old ESC, EMC and Seaby catalogues, and I think even in some of the Spinks when they took over publication, though now corrected there and by Bull.
A genuine mule would be an impossibility as the arcs and the date are on the same obverse side, though I grant that a 33 arc die might just have been prepared and then discarded, and so it is just conceivable effectively a "pattern" or two might just exist, though the absence of a recorded example these last 130-odd years would suggest not. Happy to be proved wrong, though!
Here's a shot of some blurb from the Coincraft catalogue of 2002. The Dickinson referred to is "Victorian Godless and Gothic Florins", SCMB 1978/1980 (Seaby Coin and Medal Bulletin) but I don't have a copy of those. Does any member have, and can they post screenshots? Ironic then that they kept it in their catalogue for many years afterwards!