This is the Great Planes ElectriFly 12mm Ammo Gear Drive.

For 12mm ElectriFly Ammo Motors and most other 12mm In-runner motors High-precision spur and pinion gear for quiet operation. Dual ball-bearing supported output shaft. Bolts directly to the ElectriFly Brushless Motor Mount (GPMG1250). Two different pinion gear ratios, 5.3:1 and 7.7:1.

Specifications :
Length : 1.97 in. (50mm) approx.
Width : 0.59 in. (15mm) approx.
Motor output shaft dimension : 3mm
Mount Pattern : 5x8mm stick
Gear ratios included : 5.3:1 with 12T pinion and 64T spur 7.1:1 with 9T pinion and 64T spur
Weight : 0.24oz (7g)