Here it is! One of the Color Computer 3 prototypes made by Tandy. Note how it has the expansion slot, reset, audio/video ports, serial, cassette, and joysticks ports... It also has a DB9 type connector, which is probably the monitor. Tandy probably had a cost savings to use the weird square 10 pin header connector on the bottom of the CoCo for the production monitor. Oddly, there does not appear to be a GIME!

http://www.coco25.com/coco3/