I think s.d. is some reference to a date. I know that n.d. means "no date".
Here are the complete citations I encountered:
Tennessee Wildlife Resources Agency. "Asian Carp in Tennessee Information and Images." (s.d.): tn.gov/twra/wildlife/fish/asian-carp.html. United States Geological Survey.
"Are Asian Carp Dangerous?" U.S. Department of Interior (s.d.) usgs.gov/faqs/….
These citations are in a report by the Howard Baker Center. My staff attorney says it may be Latin for sine die which means "without a date".