309 — Mp4

: Used for venting bathroom fans and dryers through the side of a house. Other Possible Interpretations

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

: A PMP-309 is an older model of a handheld digital music and video player. : Used for venting bathroom fans and dryers

: This packaging code indicates that the item is sold as a set of 4 units . 309 mp4