I have no combination code for your safe, myself. I would have to work a minor miracle to have any clue as to what it is, from where I am and with the information I have on the safe.
Sentry Safes (the company) allows you to receive their registered combination (except perhaps for older safes, like this one - I'm not sure...) if you can present a certified copy of the original bill of sale. This keeps safes 'safe' literally, by requiring proof of ownership be presented before sending out a combination.
Since you are not the original buyer, you probably cannot present that certified copy. I'm sorry but a locksmith (which is expensive, of course) is the only other option. Safes really are safe, and when combinations get lost, he result is: no opening the safe easily.
Blowing some powdered silicone 'lock lubricant' into the keylock, might possibly help... but I doubt it.
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