GM circuit numbering

[Peter Gargano]

Just wondering about the numbering scheme used for GM schematics. For example - Ignition modules to ECU use four circuits:

CKT 423    EST (Electronic Spark Control) from ECM
CKT 424    Bypass Control from ECM (sense to ECU too)
CKT 430    Reference to ECM once per ignition event
CKT 453    Distributor reference earth

Where do these numbers come from? 
Is there a "standard" colouring scheme for these circuits?

PG.
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