These 2 carburetors both had the Holley name tag and Made in USA cast into the body but the lower part of the castings are different. One doesn't have the 2 slots in the bottom and has a bulge for the air vent passage. The other has the 2 slots and the choke and throttle and choke plates don't have pins to hold them in.
I'm going to guess the one without the slots on bottom is for a bowl with separate drain. It had a center drain bowl, but a bowl nut w/o a center drain. I found a good side drain bowl.
You guessed right.
I got it going, the side drain one. Now I need to work on the other one. Glad I had the extra bowl. I've been soaking them a couple days in a coffee can with ATF/acetone mix. They come apart real easy. No heating them red hot and all that, or maybe I got lucky. May do the other one tomorrow, about time for the news.
The one with the side drain needs a side drain bowl and the bottom nut to hold it in place is the same as the vaporizer uses.