Lock tumblers are made from brass, almost never from steel. The really cheap house lock tumblers use pot metal. As long as it is made from brass, I'd say the after market is fine. I have an after market one in my 83 and it's been fine.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked