When I first got an M14, I was ignorant and bought the "best reviewed" magazines off of Midway USA.
I think they were Pro-Mag, but it doesn't really matter.
I shot with them for a few weeks. They fed and functioned well enough that I didn't have any notable failures.
One day, I had one round left on the bench, after my bolt locked open on my last mag of the day. I knew better than to drop it in the chamber, and send the bolt home, so I inserted it in the mag, without removing the mag.
Well, the bolt went home on my fingers! If I hadn't been wearing a glove, I'm sure I would have been in the emergency room.
I soon realized that the mags were not stopping the bolt by actuating the bolt stop. Rather, they were stopping the bolt with the follower.
That was the decision maker for me. Since then I have learned a bunch about identifying fake mags. I can tell a 1960's USGI from a fake now, but it isn't easy.
CMI mags, meet USGI specs, and as far as I know, nobody has faked them.
That's what I recommend that you purchase. Remove the floor plates and look at the springs and followers.
Then, someday when you find a good deal on some 1960's USGI mags at a gun show, you will have a reasonable chance of not getting ripped off.