[Drake] R4 T4 series

[k4elv at bellsouth.net]

I am new to the list so this may be an old question.  Although I have wanted to own a Drake line for the past 30 years. never did get to it.  Now's the time.  Somewhere in the past I seem to have read or heard there was a color dot coding system used by Drake to match up the PTO's for receiver/transmitter.  True?  If so, how did it work and which units were effected?
Thanks.  Mike K4ELV