“With the invention of photography, the pose continued but may have had an additional purpose in preventing blurring by maintaining the sitter’s hand in a single place. The pose is commonly seen in photographs of members of the military, with a number of American Civil War photographs showing the pose, or indicated by three open buttons on a tunic.[6]”
https://en.m.wikipedia.org/wiki/Hand-in-waistcoat
“With the invention of photography, the pose continued but may have had an additional purpose in preventing blurring by maintaining the sitter’s hand in a single place. The pose is commonly seen in photographs of members of the military, with a number of American Civil War photographs showing the pose, or indicated by three open buttons on a tunic.[6]”