The countermark is TI AV. The theory is that it stands for "Tiberius Claudius Augustus". https://www.romancoins.info/CMK-Caligula.html https://www.forumancientcoins.com/numiswiki/view.asp?key=agrippa aes
CNG lists a similar coin - they state it is a countermark applied during the reign of Nero or by the Flavians. https://www.acsearch.info/search.html?id=5682525
