A Magpul rep recently commented on M4Carbine forums that they might have a few runs of mags without date codes before the gan goes in place. I'm guessing they are trying to cut costs or something
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Anyway, with such a run produced and sold in Colorado before the ban, if they were to produce another run or two of dateless mags after the date the law goes into effect someone in Colorado could acquire some of them and it would be extremely hard to prove they are not pre-ban magazines.