For a 1977 HZ you want this I believe: 17057294. Just look for a build date number midway in 1977 for a 1977 HZ, I have seen 1787 and 2217 a few times.

There are no manual or auto carbs past January 1975, they are all the same as you need ported vacuum on both manual and auto carbs for the canister purge.

Adam, that is a US carb of some sort, probably a 1977 or 78 GMC truck. What is the 2 x digit code?