For DVR model: DS-7216HQHI-K2
Currently the old hardware and new hardware with (S) are both available: For old hardware, please use below FW DZ_K71_ML_STD_V3.5.371_191210 For new hardware with (S) please use below steps

1.  Upgrade the device to DZ_K71_ML_STD_V4.21.002_200214
2.  Downgrade the device to approved version DZ_K71_ML_STD_V3.5.371_191210

The V3.5.371 DVR firmware is applied with below products series.