Means it has a .70 4th gear overdrive. And they probably have a toggle switch or something on the dashboard to lock the torque converter.
700r4 is basically the older non electronic controlled version of what GM is using in current cars. I would guess it has 2.73:1 rear end ratio plus the .70 overdrive and probably 225-70/15 tires so it does pretty low rpms at highway speeds.