[Q] how do I find out the version of the installed bootloader?

I have a functioning but not up to date setup:
Apollo Nexus v2.0.1 which is unfortunately in the stock slot of Safestrap v3.75, no other ROM slots and previous FireOS backup leaves screen blank (boots and ADB accessible).
(The backstory to this: some update had gone wrong at the end of last year I this status is all I could manage to get to a functioning state.)
Now I would like to move to unlocked bootloader, TWRP and CM12.1
I tried to unlock the bootloader using the method with cuberHDX posted in other threads but in fastboot after
fastboot -i 0x1949 flash unlock 0xmmssssssss.unlock
I am getting
FAILED (remote: Unlock code is NOT correct)

I don't know what seems to be the problem. After the messy "fixes" from the end of last year I cannot quite remember which was the last version of the FireOS (and hence the bootloader that came along with it) that was installed.

--> Can anyone tell me how to find out the version of the bootloader?
Any other suggestions how to get out of my non-upgradeable situation? Any help greatly appreciated.