It is possible that the 1 was inverted then corrected, a few coins struck before a final fill, but then you would expect to see examples with the impression of the filled part on some well struck CHP1826 as you do with other over dates and legend repairs.
With there being a Farthing recorded I thought there might be a Penny out there making a set.
I have seen four of these Halfpennies in hand and read a Error Halfpenny/Farthing webpage identifying it as a variety some time ago, I emailed the Author Joe, but received no reply, He had four listed. but I couldn't eliminate any of these as duplicates from my own, London Coins have sold two and Zookeeper has had two. Therefore between 4-12 currently known but I am sure the numbers will rise once collections are checked through