Thanks very much everyone for your comments. Looking at the London Coins website, the common denominator seems to be the thick top shaft of the 5 and/or the little bubble on the end of the bottom sweep of the 5 - which matches the bottom of the 3 in the 1863 half pennies. I've had another look at my 1865 half pennies and found a couple of other possibles, I'd appreciate if you could let me know what you think of these?