G'day MrPharmer,
You will need to use the Medeco Classic or BiAxial cards from IC, depending on what core is in the lock. My 826 had a 6 pin Classic, and my 833 had a BiAxial. I'm not 100% on what the 831 contains, but you should be able to identify it, look for the BiAxial logo, and check the keypins. BiAxial keypins have the little tab that limits the rotation 180 degrees from the sidebar gates, facing to the left of the plug, and the Classic has them at 90 deg, facing the rear of the lock.
Be careful though, you may have to create a custom card to cut the Classic correctly, as the depth information for it that is currently in IC is incorrect, with all depths being .008 deeper than they should be. I let the fellows there know, and they have been super responsive. They are working to correct the card as we speak, but I installed the latest update just now, and he change isn't reflected just yet. This site has the correct depth / space info -->
http://www.locksafesystems.com/depth_and_space.htm#Medeco_Classic_Space_and_Depth. You can create a custom card by selecting 'Tools' and then 'Custom Series / Card' from the menu at the main page of IC. You will need to play around with some of the values to get it to accept your new card as well, I forget which ones I had to, but just do your best. I havent checked, but I suspect the depth / space info for BiAxial is proabably also incorrect, so be sure to cross-reference it before you cut a key.
Best of luck mate,
adi_picker