The door being open is a huge advantage for a locksmith. A locksmith will be able to disassemble the dial assembly and reset it to a new combination. Most likely due to the age the company that made the safe is no longer in business or no longer has records of this particular safe to help the locksmith get it unlocked. Also it could end up that the existing dial assembly will have to be "knocked out" and replaced with new locking and dial parts. There is no question you will need professional assistance to resolve this.
Dennis (cut2it)
C&onstructi&on C&onsultant
Over 40 yrs. as General Contractor for new const. & remodel - retired with time for you.