When I power on the phone while holding the volume Down - on the phone display where it shows Fastboot Mode it also shows ...
Device State: locked
In Developer options OEM unlocking is ON.
I tried turning on USB debugging and USB file transfer ... no Unlock Bootloader Screen in Web Installer.
Oh, wait, for some reason this doesn't work with Firefox - just remembered that.
Sigh, OK, I'll have to move to a different computer.