Next to the use by dates, there are letters and numbers like A3 or L1. Are these company codes and production lines?

Yes, those extra letters and numbers (like A3 or L1) are typically batch and production‑line codes used by the manufacturer for tracking and quality control.

Those codes (like A3 or L1) you see next to the “use‑by” dates on your milk bottles aren't safety indicators, they’re production codes meant for the manufacturer’s tracking and quality control. They often include info like the processing date/time, factory location and production line. 

For milk, you’ll often find codes that start with two digits representing the bottling location, followed by a dash and some combination of letters or numbers for the specific production line or batch. Those are not use‑by dates, just internal labels so manufacturers can trace everything back if there’s ever a recall or quality issue.