Date code??

Dana67Dart said

I asked the same question 3 years ago and posted numerous photos from ebay. No rhyme or reason.

Need help with date codes 742 - 8 3/4 center section

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J21 is the date code. J for September and 21 for the day. It's an early (up to 1968) axle. From 1969 they had a four numbers system.