I have a friend in NY who was asking me about it. I thought all the post-ban mags were either dated or stamped LE only. But I looked at some Glock 21 mags that aren't marked with either of those. Does this mean they are NY legal? I really didn't think they were that old.

I don't think any of the post ban magazines are marked. Standard capacity mags during the ban were.

I know for sure that NFML magazines and the ones with the half moon cut on the back indicates preban, but I'm not sure if there are other prebans that don't have that feature.

If the back of  the magazine tube has the Glock logo and the caliber marking is halfway between the top of the magazine and the witness holes, it's pre-ban. If the caliber marking is higher up closer to the top of the magazine, it's post-ban.

You'll notice that in the following picture, the three magazines on the right have the caliber marking higher up on the magazine, they're all post-ban.
(The 10-round magazine shown in the picture is actually pre-ban, not that it matters. Current post-ban 10-round magazines also have the caliber marking higher up on the magazine)

Please excuse my Glock ingnorance but how was the capacity reduced to 10 rounds while the body of the magazine would be the same dimensions as the full capacity? Also if you took a Post-ban 10 rounder, could you modify or convert it to hold the 17 rounds?

The internal walls of the 10-round magazine have ribs running down the length of the magazine so there's less room width-wise. The ribs are built into the steel liners, so it'd be more hassle than it's worth to try to cut the ribs down and besides if you did, you'd likely compromise the structural integrity of the magazines.

Thanks for the explaination! A local dealer has a G19, Gen 3 with night sights that they sold new to the present owner. Price out the door now is $450.00 only problem that I see is that all 3 mags are 10 round! What do you think, is this still a good deal and then buy some full cap mags?

If it's in pretty good condition that's a good price. I'd have to wonder why the current owner wants to sell it though, given all the ejection problems with recent production Glocks.

Even if the gun has the ejection problem, the 30274 ejector and possibly a new extractor would fix it, and the new ejectors should be available in the newest Gen4 trigger housings with manufacturer part number 30275. It should only be a matter of time before those newest trigger housings are available from retailers, maybe a month or two.