In "D" you started off in low gear and the transmission shifted automatically into high once you reached a certain speed. The shift quadrant had a "L" position for holding the trans in low gear.
Early automatic transmissions came in many flavors and oddities... I seem to recall one that you had to clutch but it shifted by itself, but that wasn't on a Chevy... a Kaiser? Geez, am I really that old?
Keep in mind that the main reason for "automatic" transmissions back then was the selling point that you could drive without shifting gears... imagine what an incredible thing that must have been? Not having to deal with clutches, grinding first gear, that column linkage getting hung up, ease of parking, rocking out of a snow rut... The first automatic transmissions were like magic to an awe-struck buying public.
It is my understanding that the early powerglides were a two speed tranmission that did not shift automatically. You would normally start off in drive which was high but you could select low to get moving faster and then shift mmanually into drive. This information came from a very good source on the VCCA.
