Parnassus wins my virtual deerstalker cap for guessing the inscription on the very first try. And Carol P. also wins my virtual deerstalker cap for suggesting the higher dpi, which allowed me to verify that Parnassus' guess was correct. Steve of The Urban Cottage gets a virtual deerstalker cap, as well. Congratulations!
This is my tracing over the 1200dpi scan. One note of interest: in graphology, the shape of the "A" in "Austin" is called a "Star T," and is a sign of doggedness, or persistence. Writers using that mark tend to finish whatever they start. It's interesting that Stewart's cousin used that shape for "A"s, and not "T's. Perhaps later in life he or she did.
Thanks to everyone who contributed to solving my conundrum. You are all History Detectives, and you each receive your very own virtual Ruffnerian Decoder Ring. Congratulations!
I did a quick search through census records from 1900 and 1910 and found three S. A. Austins, all born in the 1840s (which would probably be the correct birth date for this teenager with late 1850s attire):
S. A. Austin St. Clair, Alabama 1842
S. A. Austin St. Joseph, Michigan 1842
S. A. Austin Leflore, Mississippi 1846
Perhaps the sitter for my ambrotype is one of these three people, and lived into the beginning of the 20th century..