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The Agena controller (aka "encoder"), located on the edge of the seat on the pilot's side of the cabin.

Mike Collins (the pilot -- rookie crewman -- on Gemini 10) describes using the encoder in Carrying the Fire:

I sent these instructions [to the Agena] by manipulating a device near my right elbow, called an encoder, a little box topped by two concentric wheels and a lever. I spoke to the Agena in three-digit words. . . . The three-digit instructions always ended in either a one or a zero, and were formed by setting up the first digit on the outer wheel and the second digit on the inner wheel, and then transmitting all three by turning the lever from center to either the left (for zero) or the right (for one). The point is that the selection of the last digit was also the manipulation which transmitted the message instantaneously, so there was a built-in possibility for error, for the hand to move right instead of left, or vice versa, and then it was too late to change your mind.