The number you're looking for is a CAGE (Civilian and Government Entity) code. The presence of this number means that the Gov't has officially designated them as parts or items for use by our troops.
PMAGS should carry a CAGE code, as well as authentic mil-std magazines.
Technically, the CAGE code just means they are allowed to sell to the government and that is how the government "remembers" who manufactures what.
Commercial and Government Entity. (CAGE)
It doesn't mean they have ever passed any special requirements or anything. Only that they have gone through the process of acquiring a CAGE code so they can sell to the government.
It is useful for determining the actual manufacturer of an item though. For instance all NHMTG, OKAY and current production Colt magazines have CAGE Code 33710 which is OKAY Ind's code.
There have been some jerks that have taken junk aluminum bodies and put Colt or other manufacturer floor plates on them and sold them for way more than they are worth. So it pays to check CAGE codes if you are suspicious.
Also, the new SCAR 17 mags are all stamped 04TQ4, which is D&H Industries CAGE Code, which means although they are in a FN box, they are made in the US by D&H Industries.
For reference, MAGPULs CAGE code is 1LX50.