Hi @colinbrislawn That is a good question! I am not associated with the SILVA folks, or am I involved with their maintenance of taxonomy. But I typically, treat these ambiguous identifiers as, i.e. "unidentified" or "ambiguous".
I'll try and parse the new SILVA v138 DB that recently came out. I'll post a link to these files when I get a chance. This way they will have 6 or 7-rank taxonomy labels as I've done for the previous 132 DB files. They are available here, until I can find a more permanent home for them: