Classic/Antique Car Repair: overdrive, high speed pursuit, gear overdrive

Question
i just bought a 1964 chevy pick up.all original cab.it has a sticker by the ignition key that says"use overdrive when in high speed pursuit".where is overdrive on this 1964 c-20 chevy pick up longbody.is it usually a switch or button or do i have do something to the clutch with my four speed when i need to go from forth to overdrive when driving over 55 mph on the highway

Answer
I may be way off base here but I think that some one is messing with your head. I remember seeing a sticker like that on a Model A that came into the shop for restoration 20 or 30 years ago. However back in the day some transmission makers called 4th gear overdrive. The way to tell is that if first gear is a real granny gear, low speed for power, then the transmission is what we would have called the heavy duty and been installed in the C-20 3/4 ton package. If first gear is the one that you use to get the truck moving and then shift up to second and so on then it is probably an over drive with 4th being overdrive. I am not aware of an external overdrive being used on a 4 speed transmission. Also stop and think about it, a C-20 being used for high speed pursuit?
Brad