I found a similar number plate on a 5H dial -- also unmarked.
Then one turned up on a dial clearly marked 2AC and the number plate was marked 132-C.
http://www.classicrotaryphones.com/forum/index.php?topic=10624.msg117326#msg117326 This was apparently a standard marking at some point in time. It remains to be seen if there were 149-C or 150-C number plates. Hopefully we'll eventually find catalog or BSP references.