I’m fairly certain it varies per model year and options within each option package.
I had a 77 K-10 Cheyenne ( not a K-5 but just an example ) that had three in dash speakers only.
Also had a 85 K-5 Silverado that had the same setup.
A buddies 85 K-10 Scottsdale had the two outer dash speakers along with the two lower cab corners speakers.
Another buddies 86 K-10 Silverado came with all three dash speakers plus the lower cab speakers.
I gotta 83 K-30 Custom Deluxe with all three dash speakers.
Back when the squares were made options were available all over the place regardless of trim level nameplate.
It might help to spend some time reading through the brochure thread to get info on options for whatever particular year you are searching for.