It is possible that your grandfather’s name is improperly indexed. As you know, unusual names are often misspelled. If the funeral home is still in business they may have the record. Obituaries are also helpful. Family bibles are admissible in court for family births, deaths, etc. Also church records are available.
Even the grave marker is helpful.
In Oklahoma a “summary probate” is a simple process for the client. Many times a summary probate will put the minerals into the name of the heirs within a few months. To qualify for a summary probate one of the following is needed: -the decedent resided outside of Oklahoma; -the decedent died 5 or more years; or -the estate in Oklahoma is valued at $200,000 or less.
I have probated multiple generations to clear up title. So you could probably handle this in one joint probate.