I think it is "do" but I saw it in place of a couple given names and for a couple causes of death. I don't know what it means, does anyone?
Do is an abbreviation for ditto and it can be used for names, dates, places, etc.
It means that entry is the same as the previous one. Such as Pneumonia.
There is often information on "ditto" in the General Indexing Guidelines section of the Project Instructions.
It seems like it means to repeat the last column. Confusing as they also use " " for repeating.