If you look under the shift keys, there are spacers in the spots where the extra keys would go. The pad is actually active under it. So what you can do is pop

If you look under the shift keys, there are spacers in the spots where the extra keys would go. The pad is actually active under it. So what you can do is pop the plastic rivets that hold the metal back plate on to the plastic frame, and replace them with small screws/nuts (do a google search for "Model M bolt modification"). Once you get it apart, you can take out the spacers, and replace it with pivot plates, then you can put an appropriate key in the spot (from a dead keyboard, for example).