It is a very nice and BIG anvil, congratulation.
When I found my cast steel Brooks anvils, I did some search on the internet and didn’t find much information, and almost nothing on the forged one. The best I found was a post on this forum. If it’s not already done, do a search on “John Brooks anvil” and roll down the results to a reply from Bruce Beamish dated December 25, 2012.
It will not gives you the age of your anvil but maybe, at least, a part of the history?
The cast steel Brooks made after WWII looks “common” at least, around here. I found two of them and saw many others in the small adds or in my customer’s shop. It's the first time I see a forge Brooks anvil. I am not an anvil expert but: You found a 500Lb anvil which is special in itself, and it is from a brand we hardly find information on the internet. It is a nice find but maybe it is also a rare if not unique find?