Its good that you looked at the transfer slot exposure. Next time take it one step further and write down how many turns to close them off completely. This way you'll know the relationship between turning the screw in and the slot exposure.

That said, at this point the question is why the mixture adjustment screws aren't having any effect. The fuel to idle is getting drawn from somewhere and the transfer slot is the only logical location.
On a Holley, the transfer slot itself is the restriction (and also picks up air from above the blade). However, my recollection is that on at least some Eddys, there is an additional restriction just for the transfer slot. Even so, assuming that internal restriction is not dominating, slightly reducing the transfer slot opening below the throttle should send more fuel past the idle mixture screws.

The shape of the transfer slot opening is based on a several assumptions which may not hold true in your engine or even your carb. As far as the latter goes, even amongst Holley 4150/60s, some slots were made wider, some are cut taller and so forth. As a result the 'square' can not be relied on for every version or list number. Its just a starting point that works for most. The engine's characteristics at idle and off idle will determine where it should be. With the strong vacuum your engine is producing at idle, it probably needs less than many hot rods.

Edit: After re-reading your first, is it just that the AFR doesn't change with the mix screws, or is it that the mixture screws have almost no effect on anything? In other words you can turn them to nearly closed and the engine rpms don't drop. If the screws don't effect rpm and manifold vacuum, then what I wrote above is relevant. If they do cause the rpm response, then adjustment to AFR is probably going to come from an internal restriction(s).