i have one of those. it's a holley that's built like a quadrajet...very popular for the 4300 upgrade and chebbie people
It should have a "list" number on the airhorn (choke tower). It is likely the Holley spreadbore. There were not many models made and I believe they have been dicontinued now - though it hs been a while since I have been actively building carbs.
The 1850 is 600 CFM with vacuum secondaries and manual choke. There are newer versions, with different numbers, that have electric chokes but they are all 600 CFM.