Oil return pipes and tappet covers.

[Bob Bidonde]

I suspect that changes made to Model A parts without a corresponding changes in part numbers involved mostly castings & forgings. These two kinds of parts are subject to the pattern makers & die makers individual craftsmanship. The nature of pattern making and die making in days of old caused variations as no two people worked alike. However, it was the finish machining of castings & forgings that made them interchangeable in fit and function.

[alexiskai]

Originally Posted by Bob Bidonde

I suspect that changes made to Model A parts without a corresponding changes in part numbers involved mostly castings & forgings. These two kinds of parts are subject to the pattern makers & die makers individual craftsmanship.

You keep making this claim; it seems like you haven’t reviewed the ample evidence to the contrary, both in this thread and all over the service bulletins, that major design changes were made without changing the part number.