They are worth a bunch.
You need to check the early numbers. There are two printings of the early numbers. Some are marked 2d printing. These are worth less than the original printing.
As a minimum they are worth $5 a piece. That is what the Company sells copies still in stock for. But check our website here http://military-historians.org/company/plates/images/platesbynumber1.htm and you'll see many of the early plates are out of stock, making them worth more.
Sets of plates are worth more than individuals. See Abebooks for an idea of what other sets are selling for. http://www.abebooks.com/ Put Company of Military Historians in the author field. When the results come up, go to the highest numbers listed. These will be the sets for sale and are worth several thousand dollars.
You might do better with Ebay selling individuals, or selling to a dealer.
Your Dad should also have a set of Military Collector & Historian, the Company journal. They have value too. See Abebooks.
To Join the Company of Military Historians click here