I have a Holley which I am pretty sure it is a 780 CFM Vacuum Secondary. The numbers on it are 3310-1 and 3462 on the airhorn. Those were the only numbers on the airhorn. The metering blocks have R5271 33101 on the front and the rear has 7003 and 33110 on the rear.

Just wondering if this is an over the counter replacement or one that came on a specific car as well.

3310=common Holley replacement carb. -1 indicates a revised carb, the date code is probably 1983. This would be sold at like summit or jegs, not a GM replacement type deal.

Where or which one of these numbers dictate the date? Did the -1 come out in 1983? Did they make a 780 CFM carb in 1983? I was just wondering because this carb looks just like one of those regular 780's.

Looking at the Holley site and having owned more than one 3310 model, I think the -1 was still a 780 with removable jets as David said. Last time I looked, the model was up to a -4 and I believe starting with the -2 revision it became a 750cfm with a secondary metering plate instead of removable jets.

Holley has a numerical listing in .PDF format at http://www.holley.com/HiOctn/ProdLine/CarbList.pdf and I would assume these are aftermarket carbs and not OEM GM/Ford/MoPar units.