Sometimes "516" and "MP4" appear together in specific hobbyist or professional niches: How To Upload Video on Google Drive and Share Link
If you are converting physical film to MP4 using a device like the KODAK REELS Film Digitizer Go to product viewer dialog for this item.
You may see 516 as a timestamp or unique ID in automated recordings, such as body-worn camera footage (e.g., 516.mp4 in Incident Reports ). 🏎️ Related References