This mounting adapter allows you to fit a Bendix King KA-51A Slaving Accessory instrument into a standard 3.125" round hole in an aviation instrument panel.

The KA-51A units frequently fail because the corner "ears" of its external plastic housing break off due to aging and brittle construction material. By printing this mounting adapter and bonding it to the case with adhesive, you can extend the service life of these KA-51A units.

See https://www.thingiverse.com/thing:1578965 for an equivalent design for the newer Bendix King KA-51B instrument.