If you want marketable title, you need to record something establishing your title in Montana. For leasing purposes (where the bonus consideration is at risk), Affidavits of Heirship are used to help establish title. But an affidavit is not a conveyance, it can only reference events and other title documents to make it clear you are the lawful owners. From another perspective, you have defensible title already, since nobody can really claim ownership on these minerals, and if they are severed, you cannot lose them to adverse possession.
So, the best first alternative to an ancillary probate in Montana is to locate certified copies of what you can find to establish title: a probate and decree of distribution (or Personal Representatives Deed) in the state of residence at the time of death; death certificate and copy of last will and testament; or the like.
Prepare an exhibit with a legal description of the lands you own (so they will show up and be indexed in the County Clerk's tract index) and reference the last document(s) of record that establish title in your predecessors in title, i.e. the granting instruments whereby you claim title; then simply record the documents in the County.
Operators are constantly presented with such opportunities to evaluate risk (and either take a stand on a title opinion requirement or waive the requirment). It will usually be a division order analyst who gets the division order title opinion and is tasked to contact the owners of record for division orders and demand compliance with a title opinion requirement. I suggest you discuss these alternatives to satisfy their risk policy (usually based on the size of your interest), and to place a document of record that confirms the relevant facts.
When it comes to royalties, operators and first purchasers of production don't want to pay royalties and then have to pay them again to someone with better title and claim to the interest.
But where creditor claims are so old they are barred by statutes of limitations, there is no particular reason for an ancillary probate, because there are no creditors to protect.
Where the heirs all certify they are the legal owners of the interest, on a notarized document, and attach documents to establish their title, they make it known (to all the world) of their claim title to specifically described lands, and by recording establish their priority against any other adverse claimants.
It then becomes a matter of risk evaluation for the operator to accept these alternatives in lieu of demanding a complete ancillary probate, prosecuted in the probate court of the county where the interest is located. Otherwise, estimate 6 months and $3-5K in legal fees and costs for such a probate.
That usually prompts a serious comparison of the alternatives suggested above, unless of course you have a valuable interest that justifies the expense and effort of a probate proceeding.
I hope this helps clarify your situation (which is a common one) and the suggested alternatives.