You'll have to be a little more specific about the location and shape of the cut out you're referring to.
As far as I remember all vintage ACP cylinders are the same including the old models.
But no, no factory Ruger ACP convertible cylinders were ever cut for clips. The cylinder has to be removed from the gun to use and load rounds that are in clips. And the rear face of the cylinder has to be machined (faced off) about .015" to make space for the clips.
However, some have machined the cyl and used clipped ACP ammo in a 2nd cyl for a rapid reload for self defense scenarios by swapping out an extra cyl from a belt pouch.
With the old model, pre-transfer bar action, were the cylinders notched to allow the hammer to rest in the notch between cylinders like NAA minis? Could someone have had the notches milled by a gunsmith for that purpose so they could load six?
Now I know what you mean by "between the walls" in your 1st post...between the chamber walls. Sounds like you're referring to a firing pin groove. The cylinders with firing pin groove were used on the old models because when the hammer was down the firing was protruding thru the recoil shield and could contact the cylinder when someone mishandled the gun by not bringing the hammer all the way to full cock to complete the cyl rotation cycle before dropping the hammer.
Left over OM cylinders were used in NMs up to serial #46-11384 (1975). But NM cyls w/o the groove were issued starting at #46-09048 (1974), so usage overlapped.
NM firing pins do not protrude when the hammer is down unless the trigger is held, so the groove is no longer needed.