I don't think there's much that we can tell you.
We weren't expecting it.
It hasn't been uploaded to the UK portal.
There are no Release Notes for it.
As the V5 firmware caused chaos with the newer / stronger M-series NVRs I'd have to advise extreme caution before you proceed with an update.
Set time aside.
Take back-ups, etc.
If it was me, I'd wait for feedback from somebody who is able to test it on a spare machine first ...
I don’t recall there being a ‘c’ revision of the NI-I series (only the NXI-I). I’m away at the moment but ‘K21B1’ seems to be one of the other models from memory.
I had seen that firmware too and figured it was a mistake... probably firmware for an NXI type NVR.... anyway, I just tried loading it on the DS-7616NI-I2/16P spare NVR I have: