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My brother had an '86 C-10 w/305, CA truck & it also had the light in the center of the bezel. Maybe they were only on CA models?? Like Blue Ox said, the diesels had a "glow plug" light there, not the "check engine" Diesels also had 80 psi oil pressure gauges.
I've had an '83 K5 and an '86 Suburban that had the lights in the center of the bezel. Both gas engines. I've still never figured out why some trucks got them, but many did not.
The 84 and up C and K 10 1/2 ton gas smog truck have that light the have a ECU and a computer controlled carb the 20 and 30 we're not computer controlled. The bezel I have not found one with the light window post it here if you find one