1840 Farthing 2 Prong Trident

Hi guys I was wondering if you could help,

I recently purchased an 1840 Farthing but with the rare two prongs on the trident(unknowingly at the time). The 1839 date of the same variety is relatively common in comparison but I have personally only seen a few examples of the 1840. The coin itself is in good fine / about very fine grade and the error is very clear indeed. The thing is that I have only ever seen one for sale and that was many years ago and I can't remember what it sold for. No catalogues seem to list it and I've searched auction house back sale and can't find one selling.

Would anybody here have any idea as to its worth? Many thanks in advance, Jack.