My first set of brushhog blades looked similar to yours. Same situation: first time the area had been cleared let alone mowed. I can remember the blades tossing rocks that were almost 1' in diameter and over 10#. I hit plenty of rocks that wouldn't budge too.
I saved the mowpart.com site from another post here and hope at next blade replacement I'll be able to do better than PT's $50 minimum order. I'm hoping someone will need brushhog blades before me and will have figured out the part number (if there is indeed a replacement through that site).
Phil