Carter AVS question

Carter manual says those are to relieve vapor in the bores to assist hot starting. Only on pre-emmisions models.
http://www.imperialclub.org/Articles/AFB-AVS/Page05-reg.jpg*

There's some lists of Carter carbs by number. One posted here (FABO) as does this list at carburetor-parts.com says 3644 was for 1964 413 CID engines.
Look in the '65 Shop Manual to see what AFB was used on the 273s that year.

Your other carb has an idle air bleed screw (needle valve) in addition to the idle fuel mix screws. Originally those were used on cars equiped with two AFBs but maybe were used on other applications later on. The New AFB Carburetor (Session 121) Master Technician's Service Conference p.21

*page 5 from Carter AFB-AVS Service Manual