The SAR Trends, Tips, & Issues Volume 6, page 50 states:
"There are valid reasons why some of this data could be missing. FinCEN requests that you review your SAR reporting program and make enhancements if required. If the data (name, address, or SSN) is not available or is unknown, please indicate “not available” or “unknown” in the data box requesting the information. This is an excellent opportunity for your back office reviewers to improve your SAR reporting program."
My opinions are my own, and not that of my employer.