Many three prong to two prong adapters are actually listed. The problem is the only maintain their listing if installed IAW installation instructions. The proble is the instuctions states that the fork must be attached to a grounded screw. It you still have a two wire system like I do, technically you can not use them since the center screw is not grounded. NEC allows the run of a ground wire but the hassle it takes to run wire in old work, with a few dollars more you can use romex and get rid of the old fire starters.
