The 9N series began in 1939 and leaded gasoline was a extra cost premium and would not have been expected to be run in a tractor.
My book says the 9N (1939), 2N (1942), 8N (1948) design tractors had seat inserts as original.
I have found that valve seat recession from lead free fuel and non hard seat inserts is an exaggerated threat in low compression, under stressed engines like our old engines and tractors.
More important to keep correct tappet clearances in preventing valve seat wear/recession.
