"We were furnished with the regulation frock coat for street duty with the brass scales for the shoulders. (The abomination of a soldier as it kept us continually digging away with our vinegar bottle and whiting to keep the tarnish off. I had my likeness taken in this uniform and a fine looking burlesque it made."
--Memoirs of Private Alfred Bellard, 1st Regiment Veteran Reserve Corps
"Light-artillerymen went to the front decorated with brass scales on their shoulders, but finding an utter absence of such ornaments on the persons of soldiers who had been in action, and feeling sensitive about being known as recruits, these decorations soon disappeared. Theoretically, they were worn to ward off the blows of a sabre aimed by cavalrymen at the head; practically, it is doubtful whether they ever served such a purpose."
--John D. Billings (10thMassachusetts Artillery) Hardtack and Coffee
http://howardlanham.tripod.com/linkgr4/link188.html
This page has a discussion of differences in uniforms between lght and foot artillery troops. The illistration of a sergeant and corporal shows both wearing scales, which at a distance, and possibly in photos, could look like shoulder boards. As enlisted troops did wear scales, but not boards, could this be the case in your picture? Stan