Ruger was not the only one to manufacture "SB2" receivers. NEF started using investment cast receivers made by FN in Belgium. Hitchiner in NH also was a supplier.I did a search here and on google and didn't find the answer. If I remember right, H&R had another company make their receivers and heat treat them. Who was that company?
But now that Ruger owns Marlin,who owned H&R/NEF, and IF H&R/NEF transferred over with Marlin, they should be able to manufacture frames.... Hopefully they will revive the brand and start making the Handirifles again!