Folks,
As Robbie points out,
'40 OHV manifolds are one-year-only because of their length, as shown on the left in my first attachment.
Bronze manifolds are the same length as all later years.
And think about what was going on in the world in 1940. Copper and tin were strategic materials, for things like bullet casings. It wasn't going to be wasted upon something that is traditonally steel or iron.
But after the War, there was a lot of cheap brass scrap.
So cheap that the castings are prone to porosities, as shown by two bubbletested in my other attachments.
I process more than my share of bronze OHV manifolds, and they are invariably, as Robbie predicts, from '47s.
I have processed one bronze U manifold that was identified, and it was from a high-digit '46 UL.
....Cotten