Bob they are clearance for the strut. The Secondary Discharge are offset on the side. The rear carb AV does not have the slot, just flat cut. Basically the valve shape, size and weights are different depending on the carb. It also controls distribution in my opinion. No need to mess with them in a stock hemi with stock manifold. Just make sure the right ones installed.
Your description is a little off as the SDC is not part of the main. Only the carbs with AV have this, early AFB with side mount secondaries did not. The AV is blocking/disrupting flow for the main circuit, so you need this feed to prevent going lean. Just read what I said a few post up. The anti perc bleed and the High Speed bleed are the air source for secondary main.
I disagree....in an AFB CARB (subject of the discussion), the slots in secondary velocity valve assembly is for the clearance for the calibrated, pressed in fuel delivery tubes, attached to the bottom of the individual secondary booster venturii assemblies (bottom side of the strut), which l refered to as "secondary acceleration fuel delivery system". IF, in your pix of the disassembled AFB carb, you would turn over the secondary booster venturii assemblies, the small calibrated fuel feed tubes would be evident. This is the reason for the clearance slots in the velocity valve assembly. The secondary velocity valves are to control the air flow thru the secondary throttle bores and, likely, help with the overall air/fuel distribution pattern, when the counter weighted velocity valves and secondary throttle plates are open.
YES...the velocity valve assembly is unique to the carb and engine....most applications use the velocity valve, some engines do not.
IF, the secondary throttle plates are opened, WITHOUT the velocity valve present, a large bog or flat spot in the acceleration curve would result, b4 the secondary booster venturii assemblies start feeding fuel, similar to an inoperative or weak primary accelerator pump system. The secondary fuel delivery system is elegantly simple and is static, not including the counter weighted velocity valves, unlike the primary fuel delivery system which uses a "mechanical" pump in the carb. The secondary "accelerating fuel feed system" works on a combination of velocity pressure (air flow) and differential pressure (fuel flow) thru the secondary booster venturii assemblies. GM has successfully used this principle for years on the Q-Jet carb, as a pull-over fuel feed delivery system on the secondary circuit of the carb for when the air valve starts to open as well as controlled air valve opening rate.
I've "tuned" the AFB carb for years (Hemi as well as other Mopar applications and others) by varying the velocity valve counter weights,when present (both by lightening and adding weight) and mods to the secondary booster venturii assemblies fuel delivery orifices, to achieve the end result desired. I understand very well how the AFB carb operates. Just my opinion of course....