Here's an explanation of Rick-O-Sound™ (from an internet source):

                      "What's up with the TWO holes?"

 

"Rickenbacker's deluxe instruments are equipped with two output jacks. One jack is used with a standard cord, and is simply for normal mono use. The other jack is a Rick-O-Sound effects output, designed for use with a stereo equipped amplifier or with a Rick-O-Sound kit. These two output jacks should never be used at the same time. Rick-O-Sound can be used to produce special effects and multi-channel output when used with a stereo amplifier, a dual channel amplifier, or two separate amplifiers. Additionally, Rick-O-Sound can produce many other special effects with the use of external effects devices. Different external effects may be run through each amplifier or channel, producing many unique tonal variations.

"Rick-O-Sound provides a choice between three different types of effects at one time. With the pickup selector on the bass pickup, Rick-O-Sound puts out a lush and full bass sound. With the pickup selector in the middle position, a beautiful blend of highs and lows can be achieved, much like a clean chorus effect. The tonal separation is particularly good when using two amplifiers. With the pickup selector in the treble position, Rick-O-Sound produces a bright and full ringing lead tone. An example set-up might have a soft, full jazz sound with plenty of reverb at the bass setting in the amplifier or channel one, a loud and distorted heavy rock sound at the treble position in the amplifier or channel two, and a big pop rock stereo chorus type sound as a combination of the two in the middle position using both amplifiers or channels. Experiment with the endless unique stereo variations which Rick-O-Sound can produce. The results are surprising."