What part of no don't you understand? You can buy a new box for around $40. This will give you a set of instructions and you have a working box.
You can even get one that you can screw to the house for the same price.
Information on how to defeat locks is not given out by ethical locksmiths.
If you would really really like to open it yourself and gain the immense satisfaction that comes with it, then sequentially dial all the combinations. That will work eventually. You could get a crossword puzzle dictionary with a list of 3 letter words, or analyze the phone book and make a frequency analysis for likely combinations of initials.
You might even go farther and narrow down the possibilities buy looking at the age and ethnic group of most of the real estate agents or search the microfilm records at the library for the names of the real-estate agents/ brokers in your area around that time. Heck you might even ask some old timers in the real-estate business if they remember the combinations that were used back before the switch over.
Or go to a toy store and get yourself a Rubik's cube.