The National Archives has started posting their collection of valuation maps online. Right now, not everything is live, but more is sure to come. Visit the collection of B&O materials here: https://catalog.archives.gov/id/1180861
I was unable to find anything Georgetown Branch related at this time, but I will be checking back and will update this post when I do. Could you find anything interesting on there? I hope so! The collection really is fantastic.
Here are a list of the val maps I need for the GB.