Caterer on 44P7.2 is a period-21 oscillator composed of a caterer and the unnamed 44P7.2. In terms of its 56 cells, it is the smallest known non-trivial period 21 oscillator, although jam on 44P7.2 (pictured below) has a smaller bounding box.
Despite being composed of two oscillators of smaller periods (3 and 7 respectively), it is considered non-trivial because it has a single cell that is alive in generation 14 and is dead in its other 20 phases.
Generation 14 reveals the single cell with period 21 (in green)
Jam on 44P7.2 with a smaller bounding box of 17×16RLE: here
- 56P21.1 at Heinrich Koenig's Game of Life Object Catalogs