This tool has been unavailable for years, original part number apparently 13-1660. 3/16W-1/4BS on one end, 1/4W-5/16BS on the other. Should fit all Norton 500-850cc engines, absolutely necessary to torque the difficult-to-reach underside head nuts.
Thin-wall, heat-treated, works very well. However, one might still need to relieve the cylinder slightly where the back nut resides.
I will answer questions promptly, thanks for looking.