Looks like an older font, is it possible there was a GP or any letters after the K? It sure doesn't look like it. It could be it was discarded on purpose if it is fake, but still a cool find!
I was thinking the same thing. Spent 5 min with a toothpick to try and see if it had a "GP" or "EP" or even if there was a "P" after the 18K! But I don't see any other marks in there.
Finished tooth picking the inside and no other marks... Did uncover what's left of the gold plating on the inside. The house I found it at dates to the late 1880's... while I'd rather it be real 18k, its kind of cool that it appears to be a fake!