Tnx that got me on the right path they were hi lift mulching blades but J. Thomas is no longer in business, they were sold to Ariens which owns Stens and Arrowhead Products.
Stens has superseded the BL-724 to 362-820. They are 20-1/2" .236" thick 5/8" center hole and are 3" wide. Original blades were .250 thick. Don't know if they are fully the same design but this what Stens has superseded them to.
See what having the correct name and numbers off the blade get you. This is even with me never hearing of Mamba blades.
Source of the specs was the J. Thomas 2010 Catalog which I just dl and saved.