If you bought the rifle new in the box, it would have the sticker on the end of it. There is a sku # right under the serial number. Like mine is a sku # 811041. It's still the M&P 15T, but with differences from when it was first introduced. The sku # has changed(think it was 811040) along with the handguard and the iron sights. They used Troy Industries sights and handguards on the first generation. I think that they changed some of the other specs. on the 2nd generation.
As far as the free floating barrrel, if your handguard is in contact with the barrel anywhere other than at the upper receiver, your barrel is not free floating. A floating barrel is generally more accurate because as it heats up from shooting, the metal expands in equal directions. No pressure points to affect the way the barrel moves as it heats up or cools down.
Anyway to see a picture of what you got?