It's actually a 4-position coil switch for the new pickup. This is one of the brand spanking new ACG BZ1 Hybrid Humbuckers. It's a great little device that is, to quote from @skelf recent post in the ACG thread, " ...a hybrid humbucker in that it uses two different magnets for the coils. This allows for an Alnico single coil and a ceramic single coil in the same case. Combined with a rotary switch or 3 way mini toggle you have access to the separate coils and the coils combined. Think vintage single coil and modern single coil then both combined as a humbucker."
I rashly promised to record some sound samples but I should issue a public health warning that these will not be to same high standard as the excellent Colin Cunningham ACG demos. I'm going to blame COVID-19 but I have a feeling that a marginal difference in talent might be responsible. I'll post them as soon as possible.