IMB 84 key space saver keyboard.

I have an IBM 84 key space saver keyboard. It has numerals and
mathematical operators on the fore edges of some of the querty keys and
"Num Lock" on the fore edge of the back space key. I assume that this
was so you could toggle number locking by chording the backspace key with
something and use the labeled querty keys as a numeric keypad. I don't
think that this was done in hardware and there is no option for this
keyboard in any of the confituration programs.

I'd like to make one but can't figure out how to use xmodmap to do so.

What I'd like to do is set it up so that the right alt key chorded with
backspace toggles number locking. Once I've done that I'd like to have
the "U" key input a 4, the "I" key input a 5, etc when number locking is

How do I do this.