FamilySearch Indexing: US, Montana—County Birth and Death Records, Additional Helps
Please be sure to look at the entire image. On the birth certificates from later years, there is a field for the mother's signature. She often signed with her married name. Remember that we are to index the most complete version of the name, so if her married name is on there, you would index BOTH the maiden name and the married name in the surname field.
Also, on many of the delayed birth registrations, the same individuals are referred to with multiple names, or with alternate spellings. Again, read the ENTIRE image and always index the most complete version as well as all alternate spellings.