Oil return pipe and Tappet cover.

[RHD]

Oil return pipe and valve cover parts anomaly in parts book.
There were two Model 'A' oil return pipes and two valve covers each with their respective pipes.
I have a parts book dated 1st July 1932 and it covers just about all mechanical parts except some of the really early parts.
The book shows an oil return pipe A-6645 and a tappet cover A-6520.
I presume they are a matching pair.
What happened to the other oil return pipe and it's cover? They are not in the book. These parts are very common and found in just about all home workshops all around the world.
Is this an error?
Like to hear your comments. I have attached pictures from the parts book.