It's a calendar - I thought it was obvious given the month names on it but it took half-a-dozen shows and the person I'm watching with declared "it's a calendar".

http://www.unrv.com/forum/topic/7107-roman-calendar/ is probably the best thread I've seen on it but I've not done an exhaustive search.

Strictly I think I'm outside of the rules with this post. Sorry but I feel, particularly the images in that thread, give at least a starting place for genuine research.