While searching today, I came across a photo of Assistant Surgeon David Culbertson of the 43rd Indiana Infantry. In this photo, it shows his shoulder strap with what appears to be, an unusual insignia in the center of the field.
To me, it almost appears to be a caduceus. It doesn't look to be the typical "MS", as some medical officers would have in the field of their straps (as in the 2nd and 3rd photos). One thing that caught my eye was, the distance from the Captain bars to the actual insignia in the field. If you compare it to the straps with the "MS", the distance looks to be greater than those with the "MS".
Now I know, that not all uniforms nor, uniform accessories were regulation. There were many "non-regulation" uniforms, insignia, equipment, etc. that came to be during the War Between the States. However, the overall shape of the insignia is what seems to be strange to me. I'm just curious. Would anyone care to share your thoughts?
Below are a couple of period photos with the "MS" shoulder strap