As you know, flashing of the older firmware is not possible because of the boot secure into bootloader.
But there is a way to downgrade the firmware without flashing bootloader that can't be to downgrade. But unfortunately it will increase the value KNOX WARRANTY BIT 0x1.
So I extracted parts of the firmware for your convenience and an opportunity to downgrade or flashing individual parts of stock firmware..
AP = Android Platform
BL = Bootloader
CP = Modem (radio)
CSC = Region & mobile operator
pit = Partition table
N900AUCUBMJ5 - without system
Special tnx designgears for OneClick.exe
How to flash: