One could assume several outcomes.
1. You never have a problem with the law, justified or unjustified. So you go about your merry way with your legal magazines.
2. You have an issue, but it is unclear whether the magazines are preban or not. The po-po pats you on your head and you go on your merry way. (There are guys out there that ONLY use/own date-coded AR mags. Egads!)
3. You have an issue, and it is unclear whether the mag is preban or not. "Prove it's preban, JenkkiMike!" Now you need your receipt or something proving your old EG mag isn't a Korean one. Gootluck!
Now I believe (and this is MY belief - believe what you want and act accordingly) that if #3 happens, the DA is just trying to jam you up. If it's not a questionable magazine it's going to be caaahhhpkilluz bullets or illegal lube or something. So you're gonna be screwed in that instance no matter what. So I don't worry about non-date-coded AK or AR mags in my possession. In fact, I don't think I own an AK or AR mag that I didn't buy during the Federal ban. But how the hell am I going to prove that I bought said mags 18-20 years ago? I might have a TAPCO receipt or two. That's it. Maybe. About the only thing I think I'm safe on are my Circle-10's. I think.