Hi Nick,
its just a command line version because the E3 is not powerful to run proper Linux.
Linux as you know it is three main parts. There's the kernel, that's what Linux really is, and it talks to the hardware. Atop of that are the user-space programs, like the shell, ls(1), etc. They give you the command line that many of us appreciate. And then a bunch of user-space programs can together provide a graphical environment with windows, menus, etc.; a `desktop', like Gnome, or XFCE. Linux distributions bundle those all up together.
So the E3 is powerful enough to run a Linux kernel, but doesn't have enough memory to run a modern version, just as Intel 386 processors are no longer supported.
If I can’t put the original firmware back
That should be possible as you have the files, now without their 16 bytes per 512 bytes overhead. You need to work out how many blocks to erase based on the size of each file you're trying to put back. IIRC I pointed out a block is the unit of erasure and it's 16 KiB.