All Hydramatics were 4-speed. The shift quadrant on GMC trucks usually had 1-2, 1-3, and 1-4 positions, indicating how far the transmission would shift before holding any particular gear. 1-2 would correspond to the "L" position on most automatics. The 1-2 shift happens almost immediately after the vehcle starts moving, usually below 4 MPH. The overdrive sounds like an aftermarket unit of some type.
Jerry
