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If your mso100 is a Sentry Safe (it should be, the MSO100 is listed as the Fire-Resistant 0.8 cu ft. Sentry Safe), then you must get the combination from Sentry Safes Inc. again, following their procedure. (Please be advised that a safe is just that: if well-designed, there is no fast way into it without the combination/code/key.)
is the page that instructs the owner how to go about getting the combination back, for the safe. I hope that this advice helps.