The string sometimes appears in automated log files for video transcoding (e.g., /storage/6563-3262/.../VID_...mp4 ), which may be where a filename like "b6326.mp4" originated in a technical sense.
Some search results link "B6326" to documentation for the A46 Newark Bypass project in the UK, where it serves as a location or reference code. b6326.mp4
It featured a strong MPEG II decoder for its time, making it a decent budget choice for playing DVDs even if it couldn't handle high-end games. The string sometimes appears in automated log files