[StanJ]

As far as I know, all Holley carburetors have some sort of fuel metering orifice in the curb idle/transition circuit. Conventional circuitry Holley 4150/4160/2300 series carbs (not the "reverse idle" emissions-spec units, which are different animals entirely) will have them in one of three different places in the metering block:

1. In the "Y" intersection leading to the base of the idle well

2. Just underneath the idle well caps

3. In the idle circuit cross channel connecting the idle well to the face channel just in front of the idle air bleed passage (this is the default location for the Holley HP and Ultra HP carbs, as well as the QFT stuff).

Holley's 3-circuit "Dominator" carbs are also slightly different; in pre-2002 models, the idle circuit feed restriction is built into the bottom of the idle feed tube pressed into the metering block main wells, later production carbs have the idle feed restrictions in the idle circuit cross channel as with the HP Series carbs.

Now, if your question is actually about Holley carbs having replaceable "screw in" idle feed restrictions, besides the Ultra HP billet aluminum metering blocks, the the only metering block to come from Holley with them is Part# 134-276, which is a service replacement upgrade for the HP "Dorton Series" 2brls.