Well, the box has the number 2-256, under which is printed "(7890)". Parts guy didn't recognize either number.
The instruction sheet, which I found in a file later, says the carb is a model 2280, list # 7890. A Holley 2280 was the factory carb on the 318 in the '77 model year, so, at worst, I can get a '77 318 carb rebuild kit.
