If your loader has curved arms as apposed to strait arms with a welded joint to get to the bucket your loader was built by Ameriquip in Wisconsin. The strait arm loader my have been built by them also. The problem is even if they did build it is I doubt if they have built buckets for it in the last ten years. But they might know who else used it, since Montana is gone.
Short history, in the old days the arms and cylinders were pinned directly to the bucket nobody ever changed the bucket so that was fine. Then customers wanted to be able to change buckets or use pallet forks. All the manufacturers built a proprietary Quick change system, which is what you have (I think John Deere had one similar) and today most use Skid Steer Quick Attach.
So today I think your cheapest option would be to fix your old bucket
Next would be have one custom built or have one modified with your brackets.
And the most expensive would be get a SSQA bracket that pins onto your arms and cylinder replacing the bracket that you are measuring, and then buy a SSQA bucket that is cheaper then the custom built bucket. Definitely the most expensive option but if you see yourself wanting more attachments in the future it might be cheaper in the long run.
Sorry for the doom and gloom
Gary