Loyal, you must really want to know! Somehow, you managed to post the same thing 3 times! If you go back, I think you'll see a 'delete' button in the upper right corner of your posts and you can delete 1 or 2 of them.
Here's one website that I found that calculates the weight for you, if it's carbon steel:
http://www.portlandbolt.com/steel-plate-weight.html
Here's a table that lists weight in pounds per foot:
http://www.engineeringtoolbox.com/steel ... d_774.html
...There are probably others....
I don't blame you for waiting for trailer lights. It's probably illegal to pull it without them. It sounds like what the original builder did was really nice, albeit probably a bit on the heavy side.