EIAJ connector

The entire series of connectors has a built-in retention feature.

On types 1, 2, and 3 this involves a bevel on the barrel near the tip, while on types 4 and 5 the tip of the insulator ring has a larger diameter than the metal barrel.

The design philosophy is that higher voltages should be supplied via larger plugs to prevent equipment damage.

[1] For instance, this document describes the interface between a car cigarette plug or RC-5321 plug and the powered device, the polarity symbol used (JEITA CP-1104A), This is a unique-looking barrel connector, apparently intended for providing a DC output jack (most DC jacks are used for power input).

According to Hosiden, this is a 12V/24V barrel connector designed for electronic equipment used in cars.