Thank you, Ron (AC7AC). The Radio Shack P/N 274-284 plug does indeed fit the STEREO/MONO jack on the back of the K3. The Radio Shack package shows it as a 1/8" Stereo Phone Plug. Nevertheless, it fits the K3 jack without wiggling around. I find it very hard to eyeball the difference between the 1/8" and 3.5mm plugs. Roy Morris W4WFB