Base timing vs. ignition timing
 

This should make more sense

Seloc And Clymer are notorious for putting several models in the same
manual and not always giving proper reference when doing procedures. It's
better to
have the OEM manual for the engine you have.
The OP could have a Thunderbolt 4 or 5 depending on year. If I recall as
long as RPM was 550 to 600 the base timing could be read without grounding
or shorting wires.