Two pictures at right show what a rear leaf spring shackle should look like, having two cross studs welded to one of the side plates. The second side plate seats against shoulders on the studs before being securely bolted in place. The picture below shows the result of bad design from someone who didn't understand the real world function of a spring shackle.

This one was built with the cross studs lightly knurled under the head, and press fit into the side plate with no welding. Material for the stud is also sub-standard steel,soft with no heat treat. During first assemble to the car, two things happened. One stud rotated to strip the knurled spline, while the other one stripped the male thread while the hex nut was still good, and all this with less than 40 lb-ft torque.

Being structural parts, high strength is required. High carbon steel for the plates, and grade 5 studs and nuts should be good. The studs should be 1/2" with shoulder and 3/8" threads and nuts. Just knurling the shank of the plain 1/2" stud is not going to raise the metal enough for a structurally strong joint, which is why the male spline has stripped out. That joint should have the head of the stud welded to the shackle plate. But then the soft metal of the stud also let the male thread strip as the nut was tightened, so this is just garbage parts all around.

The fixed plate needs 1/2" holes so the 1/2" stud can be slipped in or pressed in with the head welded in place. The removable plate needs 3/8" clearance holes to slip over the 3/8" threads, to be tightened up against the shoulder on the studs. In assembly, the plates, studs and nuts need to mate as a single ridged piece with fixed space between the plates when the nuts are tight. That shackle assembly moves as a single piece in operation with nothing allowed to wiggle in the joints other the the resilient bushings.

Bottom line is, when the knurling strips, or the threads strip, the parts are junk. Make the report to the vendor, get your money back, and do make the request for the vendor to stop sales of the faulty parts. One vendor hasalready refunded the money,and the next must do the same. See if you can get a response from the vendor if they will stop sales or not. It will be up to the vendor to deal with the source manufacturer.