Allison Labs 2ABR Variable Inductors

I just bought 2 Allison Labs 2ABR low/high cut LC filters with variable inductors. I know nothing about these other than what I can gather from pics like this:



This particular pic is not from a 2ABR but it looks like the variable inductors are the same. Apparently the inductance can be adjusted by as much as a factor of 3. Does anyone know how these variable inductors work? Is that gear mech just moving some kind of core in / out?

Would anyone happen to have a schematic? It looks a whole lot like a Pultec HLF3C to me.