As Jake mentioned last time, SII has upgraded popular 6R15B movement to 6R15C. If you use TMI products, that's NE15 to NE15B. The upgrade includes:
1) Add one jewel on main plate, barrel hole (6R20 has that jewel already)
2) Modify ratchet sliding spring
3) Modify date indicator maintenance plate
4) New balance wheel
5) New barrel
Looked carefully, adding jewel and modify ratchet spring are all related to upgrading of barrel (to higher power reserve, so higher torque). Modify date plate is due to widely complains of date misalignment. SII upgraded balance wheel might aim to increase long term stability. The previous 6R15 all have difficulty maintain long term (1-2years) stability in accuracy.