Exactly right, Nerwen, and well done Urwen for guessing it was one of Bilbo's gifts 'speaking.'
A 'pen and ink' is indeed Cockney slang for a stink, but that wasn't Bilbo's meaning; nor would such a meaning be known in the Shire, as far as I know. (Maybe more Shagrat and Gorbag's style?)
And a pen and ink bottle could, as Nerwen said, be used to write further messages.
I never thought, though, to get such a different interpretation as Celebrimbor on a pole! That was an eye-opener!