Did you buy it from Rivendell Bicycle Works, or did you buy it second hand?
Do you think it was manufactured in 2015? or do you think it's a lot older than that?
If it's a lot older than 2015, then it's probably built by Toyo
The best clues are probably the rear dropouts and the serial number. Remove the rear wheel and look at the rear dropouts. If "Waterford" is imprinted in the rear dropout where the wheel used to be, then it's a Waterford. If not, then it's probably Toyo. The Toyo serial numbers on Atlantis frames were always "AT####", as I understand it.
Bill Lindsay
El Cerrito, CA