I have b930
I need b300 for dungrade from b331 to b930
If you have b300 link plz send that for me
Sorry for the delayed response!
B300 seems to be for Mate 7 not for Mate 1.
If you have Mate 1, my recommendation would be the latest KangVIP ROM which is based on B331 Stock ROM.
Even better: One of the fine Custom ROMs by dev mrAlmid (OmniROM, BlissStalk ROM, CyanogenMod 11).
Bliss and CM11 had been revised in 2017.01.
You can find these ROMs and usb drivers as well on my MEGA.