Personally, I’d favor moving away from the practice of defacing the magnifying glass in OSM’s logo. That’s a common practice among OSM’s local chapters and thematic user groups, but OHM is much more ambitious than a user group for OSM. An OSM-based logo is also nearly indistinguishable from the OSM logo at a small size, such as a favicon. If we do want to maintain a visual connection to the OSM project, we could use the OSM logo as a silhouette, such as in the Public Domain Map logo, but people who encounter OHM before becoming familiar with OSM may not intuit the connection anyhow.
I think there’s value in designing a single logo to represent the whole project, regardless of time period. Even though no single timekeeping device was used throughout all of human history, the logo can still symbolize the practice of historiography by depicting a well recognized device. Or we could go more abstract with something like a timeline.