The vent will be on top of the axle between the differential and backing plate. On my Chevy trucks it is on the left (driver's) side. It will either be pressed in or threaded ( 1/8 pipe thread). My '42 did not have anything but a hole when I bought it.
Kent


1937 Chevy 1/2 ton
1942 Chevy 1/2 ton
1947 Diamond T Model 509
1951 Chevy 1/2 ton
1950 Chevy COE Model 5700 ~ "Barney" ~ And more pix