in my Great Grandmother's United States Marine Corps muster roll record the following information appears to be provided:
055 1-17 & 19...30, D, FPOSF; 18, only, sk qtrs.
How do I interpret this?
I don't have a full answer, but this is too much like an answer to be a comment.
FPOSF is Fleet Post Office, San Francisco, where she would have received her mail.
1-17 & 19-30, combined with 18 only, look like days of a month.
On the 18th only, she was "sk qtrs" or "sick in quarters."
An answer to Marine Corps Muster Roll abbreviations may help with some of the rest.