Most of these Grand Tour Roman cameos have a name attached in Latin of the imperial personage depicted. The fact this one is blank probably indicates it's a generic emperor or Roman. Mary Beards next book will tackle these sorts of questions, it's about Roman emperors images down through the ages. She did a series of Mellon Lectures on the subject. Here's Part 1: The other lectures can be viewed here: https://www.nga.gov/global-site-search-page.html?searchterm=mary beard&category=Audio/Video&pageNumber=1&lastFacet=category