Press the open/close button so that the drawer is out.

Now press the stop button (I know the unit is already stopped)

Press 1 9 9 9

(Again you'll probably get a symbol bottom right on each key
press indicating you can't do that - Circle with line through it)

Hopefully you should now get a screen similar to this appearing

OK the DVD-ROM REV is the one we want in this case it's blank
it's normally above the CS4955 writing.
Yours may also be blank or it might say 1.7B or something else again let me know