2015 is an "evolution" of the platform. The engine is the same, but the body, frame and ASA/reg, and foregrip are different. The 2015 also comes stock with the Redline board, two piece barrel, and the nicer feed neck that's an accessory for the 2014 and prior Axe.
It also bears mentioning that Empire is still building the "original" Axe platform, but as a base gun only. You may well find a 2015 Axe that's had a Redline board added, and the accessory neck, and an upgraded barrel, but the "Pro" tag was only officially used for the new platform guns. It's been used by others for original platform Axes but not by Empire.
Visually, the two guns are distinctive.
2014 and prior - this one happens to be a limited edition with the older Redline and accessory neck, as well as a Freak barrel.