Many go on the assumption that no external/internal date stamp USGI mags are good to go. YMMV.
There are others that just don't care about the AWB/ mag limit...Saw someone at the range with a bunch of 30rnd Pmags...I just chuckled and said, "those look cool".
There are also a lot of police that have no idea that there is a mag limit...I have had police that "know the law" say I was fine so long as I had a CL-A NR license and didn't understand why I had 10rnd mags.
A few years back I wanted to pick up some more mags for one of my Glocks, so I went looking on Gunbroker and found an auction that was specifically calling out that it was selling 10 round mags for ban states - and bought 10 mags.
The package shows up a week or so later - with 10 brand new 15 round mags in it. I called the guy up as fast as I could and asked him why he 1) sent something different from what the auction specifically stated - and, 2) - if he knew what the MA laws were.
This started whole conversation with him trying to tell me that the mags were not illegal in MA - that he sold lots of them out at the Big E gun shows in Springfield - and that he had multiple state troopers tell him the mags were legal to own in MA. I had to tell the guy what he had essentially done was send me 10 felony counts in a box - and if he continued to send out illegal mags to MA residents that he might find Martha Coakley crawling up his butt at some future point in time. I also had to tell him that the cops simply didn't know the law in many cases. He still didn't believe me - and didn't seem to want to exchange the mags. So I had to dig up the relevant laws - much big thanks to NES - and send them off to him.
Finally he agreed to exchange them and I (finally) got the right ones.
Then there's the guy I bought a Glock from - who told me the story about how he had also just sold an FS2000 - because he went thru a shit-show at the range with it one day when a guy who claimed to be a cop was all up and down him about not being able own that rifle "because it was a machine gun". His claim was the guy ordered him to stop shooting and basically held him there while he called around to captains and lietenants and so forth trying to get clarification on the law. After that experience he basically decided to just ditch the rifle and avoid the hassle.
Yeah - my experience is cops just don't know the law - in BOTH directions.