From 2013 onwards, all BTR frames feature a straight 44mm head tube. All frames are only compatible with headsets with an external lower cup. So the S.H.I.S. code is ZS44/28.6/H8 | EC44/40/H12 for a headset that accepts a tapered steerer. The S.H.I.S. code for a straight 1 1/8″ steerer is ZS44/28.6/H8 | EC44/30/H12.
Belters made before 2013 are only compatible with IS41/28.6/H9 | IS41/30/H1 integrated headsets.