A 48 with a three-speed would have a column shifter. It sounds like it has an SM420 synchromesh 4 speed which was introduced in late 1947. To put it in reverse (from neutral), move the lever to the extreme left (against spring tension), and pull straight back. You can never discount the possibility that someone replaced it with a 3 speed. The 4 speed will have a 6-bolt pto cover on the side of the transmission.