Note that this shaped tool is for assembly of engine, NOT DISASSEMBLY.  Once the nuts are torqued and they've been installed for a long time it takes MUCH more torque to remove them.

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.