This is a pack of replacement plastic servo gears that are used in the Spektrum DSP75 servos. If you strip the gears in a servo, it most likely will need to be replaced one of these small plastic gears. There are five gears in the package.
This gear set is identical to the EFLRDS751 gear set, which fits the EFLRDS75H servo used in the Blade 400 helicopter.