A lot of times the number after the model just signifies the tier level of the bike and the groupset and wheels that come with it. But googling, It looks like it might be the frame differences that they made back in 2016.

https://www.bikeradar.com/features/a...-race-chassis/

If you look at the new Felt FR's and go to the specs, they say for the top dollar ones that the frame is the FR3 today. So not something you know unless you look at the specs.