It's true that the rocker arm oil came off the low pressure side up to and including 1939.

For 1940 and after, the rocker arm oil came off the HIGH pressure side.
But... it ran past a bleed off hole which ran back to the low pressure side of the oil distributor valve... and then through a metered orifice... and then into the line up to the rocker arms.

This change was made when the change to the newer style oil distributor valve with the sheet metal cover was made, and I suspect this was done to make lubrication more reliable and less likely to plug in the line up the rocker arms.