Hub assembly missing spindle nut

The pin at the top of the circled part pokes into a rubber ring to lock it from working loose. This is a very uncommon design. There is a more common double pin made of a folded piece of metal, about 3/8" long. If that is what was used, you have nothing to worry about.

The little hook at the top of part number 7 goes into the rubber ring in the center of part number 6. Your mechanic installed the more common hook that does not include that half ring. The ring is there to provide something to grab. The pin that was installed is removed by grabbing it with a pair of pliers. That part has been used since the 1960's without issues, and it will work just as well on your truck. There is a groove on the top of the spindle that parts three through nine slide onto. The hook your mechanic installed sits in that groove, and it cuts into the rubber ring on the lock nut, (part 6). That is what prevents the lock nut from working loose. Both styles of hook do the same thing.