What is an "Air Core Coil" chakster? I’ve never heard that term.
The term is old
It's the opposite to an Iron Core
"Compared to the iron core type, which is the mainstream of MC cartridges, the air core type is not affected by magnetic distortion, so a natural tone color and precise sound field expression are possible" .
Another Air Core Coil type is an old Fidelity-Research FR-7fz for example