Help with hallmark on 19th century watch?

The attached is a mark inside the case of my great grandfather’s pocket watch, made sometime in the 19th century by John Hornby in Liverpool. I’m assuming it’s silver but my friend who has a book of hallmarks can’t find it. I’m mostly curious just to get a date on it. There is a receipt tucked in the case which says it was last cleaned in March 1899 but it still runs just fine! Great Grandad was a ploughman in Suffolk, England, and even though it’s not a particularly fancy watch, I imagine it would have been quite valuable to him.

Any help appreciated.

Hallmark outside case.jpg

Update- after following other links I found on this site, that looks like a Chester mark and I photographed it upside down. So the ‘O’ would be 1810. But what does the NI mean? Am I close?