Quoted from BrianBannon:
Most of the metal used in classic Sterns looks like a "pot metal", I am guessing mostly zinc, and that impacts a number of parts. Worst part I have run into by far are those lousy lockdown receivers, followed by the first generation flipper baseplates.
AKAIK, Stern used two different styles of lockdown receivers. One style used the ball and socket type of affair. The later style moved to the slotted style of receiver. For a long time, I figured Stern thought the slotted style was better the ball and socket and this is why the ball and socket was replaced. However, I do have a ball and socket style lockdown bar and receiver and I like it because the ball is adjustable. If your lockdown bar is a little loose, the ball can be adjusted.
With the ball and socket style, Stern had to produce the ball with a threaded shaft. The receiver unit has to be drilled and threaded. And a special lock nut had to be produced. And then someone had to thread the nut onto the threaded shaft of the ball. Then the ball and lock nut had to be screwed into the threaded holes on the receiver. And then it had to be adjusted.
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The lockdown receiver assembly that replaced the ball and socket style with the slotted style was just one piece of metal that is spot welded to the lockdown bar.
Gone was the the ball that had to have threads machined on. Gone was the nut that had to have threads machined on. No machining was required on the new receiver assembly.
I'll hazard a guess that the later slotted style of a receiver assembly was a cost saving measure only. The slotted style worked OK and was cheaper to produce.
Can I prove all of this? No. But in my mind, a cost savings was the only reason for Stern to change styles.