From my Experience in downgrading Samsung Phone, Look at the digit "1" before the last four alphanumeric series
AP: G935FXXU1DQB3
CP: G935FXXU1DQB1
CSC: G935FOXA1DQB1
if you need to downgrade a phone, that digit of the Firmware in the phone should be same with the New Firmware intended to be use.
i hope u understood, pardon my english