This is the Great Planes ElectriFly 4mm male bullet to 3.5mm female bullet adapter.
Features: Connects a motor with 3.5mm Male Bullet Connectors to an Electronic Speed Control with 4.0mm Female Bullet Connectors
Includes: Three Great Planes ElectriFly 4.0mm Male Bullet to 3.5mm Female Bullet Adapters
Connector #1: 4mm Male Bullet
Connector #2: 3.5mm Female Bullet
Weight: 0.08oz (2.14g) each adapter