Same problem - I have the key but no combination.

You can't. This safe has a rotary combination lock and a failsafe key. The key is your insurance if you can't get in.

Call a local locksmith, for what will probably be destructive entry unless you have the serial number of the safe. A GOOD locksmith may also be able to impression a key for you. According to this you can get keys on proof of ownership.

https://www.lockpicking101.com/viewtopic.php?f=36&t=64109

How can I get the combination for our brinks fire safe model ...

Answers https://www.answers.com > Hobbies > Guns & Weaponry
Sep 15, 2023 - Get the model and serial from your safe as well as the key number. Call Honeywell @ 800-223-8566.

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Give them both numbers. Normally the longer number is the serial. another workaround is to get another key made locally.

https://www.google.com/search?q=1994+gmc+jimmy+back+glass+won%27t+open

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notify brinks or a locksmith

The key is the over ride to open it. If it is a dial combination, turn the key to unlock, then turn the dial to withdraw the bolts. IIRC, the last turn on a combination safe is clockwise. But according to page 2, if you only use the key, it means you are keeping the dial at it last position. From what is says, you now need to open it again using the combination that was given to you.
https://firstalert.my.salesforce.com/sfc/p/#o0000000bgz6/a/3l000000XrEk/q9_eBXnzfEFRbQEkPgJge9p9WPx.DFjuECqJ70zwArc

https://support.firstalert.com/s/article/brinks-safe-manuals

and also

https://brinkshome.com/smartcenter/a-brinks-home-security-box

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since manufacturer cant assist you will need a locksmith

Perhaps a "fun" person, did give the combination lock a swing?
I hope someone still knows the combination. You will need it.

Contact brinks they may have a master code to unlock your safe...or try a good lock smith,they may have access to the code.

try taking it to a lock smith it will cost about 15.00 to 20.00 depending who you go to they will make a key or find the combination call a round . good luck

go to honeywellsafes.com apparently brinks was bought out