DADS50, That item is the factory lock, it's a cast iron piece and locks on to a special nut on the 'J' hook. I believe this lock was used on later years also. The locks are getting hard to find and expensive.
FOUND PICS-in my old 41 Salesmens Truck data book, pics of light duty and 3/4 ton tire carrier called Monorail carrier. Also checked in my 42 parts manual, no pics but part numbers are the same for 41 and 42, see attached Webshots link.
http://rides.webshots.com/album/582371665xpYwSg
