Hi GearRat,
John beat me to the punch. I've been researching my casting numbers and have run across a lot of info. The following site has some info on the number reference to year.
http://www.telusplanet.net/public/bboogaar/engine.htm
Your engine looks to be a '45, or at least the heads with "59A" and the following numbers usually indicate a part number.
Looks like you have a fun project, cool to be handed down in the family and hope you get a chance to pass it on eventually.
Good Luck!