If you aren't looking for a hardware component, "309.mp4" is also a common filename for digital media:

: This packaging code indicates that the item is sold as a set of 4 units .

: Several short videos and animations on Vimeo use this numerical title (e.g., 309.mp4 by The Visual Brand ).

: A PMP-309 is an older model of a handheld digital music and video player.