That key is actually part of the standard 99/4 overlay. It makes it easier to pick up the overlay. Looking at your console, it looks like you have an overlay in place--and the key is probably attached to that. I have several such overlays in my collection, most of which have the blank keys on them (later issues didn't have the keys, even of the same overlay, as the change came somewhere about halfway through the 99/4 lifecycle).
I have a separate overlay that I got when I bought the 99/4; it does not have a cut-out for that key, and I did not include that overlay in the photos. Having said that, and after reading your post, though, I'll have to check my 99/4 when I get home from work today to see if the overlay of which you speak that includes a blank key glued to it has actually already been mounted permanently to my console. When I got the console, I just assumed that the option commands printed above the keys (Redo, Clear, Ins, Del, etc.) were actually printed on the console.
Thanks for the info, and I'll let you know what I find.