perhpas those markings on the speedo are meant exclusively for manual trans and they didnt bother to remove them for the automatic?
the beaters i have driven for just transportation all made it clear thhat one ought not use any other selection than "D" on the transmission, unless posed with a steep grade, etc. However, my '68 GTO has an automatic that was clearly meant to be shifted either by user or by the trans. The 300e shifter seems notched in a manner similar to the old GTO, as in steps without an "idiot button" to prevent it from moving, hence implying that it ok to manually shift the automatic, like the transmission was designed for it. However, I dont want to screw around wth it unless im sure...