Have an old SUPRA-C lockbox but forgot the code! How can I figure out the code to open. I am a Realtor

You might occasionally forget the lock box combination and be unable to access the contents. Fortunately, there is still something you can attempt to figure out the Supra lock combination. In this article, we'll walk you through the simple methods needed to break into a Supra combination lock box. https://revolar.com/hack-supra-combination-lock-box/

Wow!! This brings back memories! The answer depends on time and value. If you're in a hurry and it's important you get access quick, it most likely will require forced entry and will damage destroy the box. If you have some time and don't mind investing money into the box, a locksmith or check for a hobby locksmith group near you might be able to crack the code for you.

If you need access now. You can start with hitting the black button to see if it will move down far enough to open. I've been able to do that many times. But if that don't work you will need to be more aggressive. Place the box on some very solid, concrete preferably. Get some safety glasses. Hold the box on it side and carefully, hold onto the shackle and hit it with a medium sized hammer. Be careful, parts can fly off when you hit it. I've even wrapped them in tape before. It normally takes a couple of good hits before it pops open.
Good luck.

Pick the lock, Manufactures reset or hire a lock smith.

https://www.hunker.com/12399239/how-to-open-a-lock-box-with-a-lost-combination