[Milsurplus] BC-375 neutralizing cap?

[Ian Wilson]

Recently picked up a 4-pin socket (VT-4C type). Included was a
neutralizing capacitor. This one appears to have 3 connections, which
puzzles me.
The central plate is fixed, while the two outer ones are adjustable
relative to the
centre (each plate is a bulbous circular disk).

(Since writing this I took a look at the BC-375 schematic, and don't see any
neutralizing caps. But it's a triode rig .. did I miss something?).

Looking for enlightenment.

73, ian K3IMW