[Coco] CoCo 3 RGB Pinout

[Michael Graham]

I have a question about the CoCo 3 RGB pinout for those of you who might 
know.  All the pinout sources I've found list pin 10 of the connector as 
'no connection', but upon investigation of my CoCo 3 circuit board (and 
the schematic I found online) it is in fact connected to pin 13 (PB3) of 
the 68B21, with a pull-up to +5v.  What purpose is this pin actually 
supposed to serve?

Thanks,
Michael