To read the lot codes: first letter indicates country of origin factory, second and third characters (first two digits) indicate the year of manufacture, so 20 would be 2020, next 3 digits indicate how many days since the start of the year they were made (so 330 would be 330 days since Jan 1, 2020). Last one is the machine that made it.

If anyone can chime in on where that 5 year rating comes from that would be great (I know for a lot of drugs the maximum safe time is from tests done by the manufacturers and in their own interest they only test for a few, maybe 5 years and when the US military tested some substances they discovered they retained most of their potency for much longer). As I wouldn’t be surprised if it’s a case of 3M only warranties them that long and under proper storage they can last longer but at the same time I understand there is something of a static charge in parts of the mask that does the heavy lifting for particle trapping and that may dissipate over time or other materials may suffer degradation. I know it’s important to keep them sealed in the wrapper packaging until time of use for instance.

The things to look for as I understand are: they don’t smell strongly off when opened (outside of packaging smelling doesn’t matter), the adhesives aren’t breaking down (nose pads coming off, melting), the straps don’t snap on first use (probably least consequential but it could be a sign of bad storage conditions and stresses to the rubber/elastic and hence the rest of it) and they fit test properly. The straps on these ones aren’t that great anyways, I’ve had store bought ones of this model break on me after the first use if I pulled a little too far trying to get them out and over my hair without tangles.