when did GM put catalytic converters on Sierra 305s?

[baradonai]

I bought my 1977 GMC Sierra with a 305 from a guy in Alabama about 4-5 years ago. Other than change oil,plugs, wires and brakes it's just the way I found it. i've passed emissions every year...except this year. All of a sudden they tell me it's missing the catalytic converters. I'm suspicious. Did these trucks even have cats? LMC doesn't list them in their parts. The two doofuses in the testing facility looked like couldn't make change from a twenty without a compter. Fumbled all over the truck looking for numbers, couldn't open the hood without my help. Frankly, I think I'm being railroaded.