There is a steel hook that threads into the blade and locks on to a sprung loaded bar in the stock. Over years of use, the hook takes a beating and starts to split/straighten out.
I'm sure that these hooks are available and are easy to replace. If the hook mounting in the blade is damaged, epoxy filler will cure it.
Prindle rudders are pretty straight foward and work really well if you give them a bit of maintainence/lube now and again.