Right enough BF, that Chilton/Haynes manual is crap!
Good chance the hex headed job in the sump is the thing. I've been more into Alfa's, and they often have an oil level sensor in them - it's a pretty crude thing. Bolts in to the bottom of the block and extends into the sump as you'd expect, the sensor itself is a small bi-metal contact and a heater coil. The thing gets volts put on it briefly at startup which heats the coil, if the oil is low, this allows the bi-metallic to get hot enough to open the switch, which triggers the warning lamp.