I thought this was a switch... but it isn't!

I had expected the red thing on the rear of this Amperite RB ribbon mic to be a simple on-off switch:

Amperite RB - switch on rear

In fact the knob is connected to a cloth-covered metal plate which slides up and down, blocking the rear of the mic. This is a mechanical method of altering both the pickup pattern and the frequency response.

Inside the Amperite - sliding plate, blurry photo :-(

When open, the mic will be a normal figure-8 pattern. Closing the vent will reduce pickup at the rear, and also make the mic sound brighter by reducing the proximity effect.

Open and….

… closed

The mechanism is not as sophisticated as, for example, an RCA 77DX, but at least they made an early effort to offer engineers a new flavour.

Well done Amperite!