PRND21... a variety of gear indicators from the 1950's and 60's

I think it's interesting to find that there were more than a couple ways for an automatic to name the gears. What was the S in PNDSLR? Was O in PRNODL2L1 for overdrive? Must have been. One of these has D2D1 and then L.... and another has DL2L1... what do you suppose the difference is between 2 drive gears and one low, versus one drive gear and 2 low gears?