They have both mechanical and vacuum advance. They also have what is called an ESC (Electronic Spark Control). The only thing the ESC does is control spark knock. You notice the distributor plug has 4 pin plug on it.
It also has a knock sensor in the engine block (in front of the starter). When the knock sensor detects spark knock it sends a signal to the ESC contoller (mounted behind the glove box, above the heater). The ESC will retard the timing to stop the spark knock.
As a side note, I have removed all that mess on my 85 C10. I installed a MSD Pro-Billet HEI.
Hope this helps,
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Mike
1985 Chevy C-10
Mike's 73-87 Chevy Pickup Site