Gliders by the dozen is a methuselah with a lifespan of 184 generations. It was first reported in a larger form (shown below) by Roger H. Rosenbaum in Lifeline Volume 3. The pattern shown to the right is a predecessor of generation 1 of the original form.
Its name arises from the fact that it emits exactly 12 gliders before stabilizing. It is possible to reflect some of them to clean up the junks and release the other(s), which then becomes an over-unity reaction capable of driving glider guns. This is the core mechanism of cyclotron, a period-874 glider gun built by Dean Hickerson in 1992.
The stable pattern that results from gliders by the dozen has 124 cells and consists of 12 gliders, 8 blinkers, 4 beehives and 4 blocks.