No nylon teeth timing gears in my experience after 1971, this car should have had an all-metal gear and chain set. Check the date code on the big sprocket for the chain set and you'll find out if it's originally a GM one and how close to the engine build date it is.
AT5 is probably a line inspection stamp. They inked them on in the late '70s instead of the paper tags from the earlier cars.
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