I have seen this tintype from my collection posted on Pinterest as a post mortem. Why? Because the twin on the right has the base of a posing stand behind him. I have often seen a comment "If he wasn't dead why would he need to be held up by a stand?" One of the reasons I bought his tintype was because it illustrates so well the reason for head brace. The twin on the left has no head stand and he is slightly blurred because he moved his head.
9Gag's Gentlemen's Guide to Fragrances...
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A home made developing sink for the darkroom
Head Brace Stand
A 19th century photographer's neck clamp/stand for studio photographs. Cast iron with three spread foot base
This is an except from a later 19th century photographic publication by Wilson. It says that the increased sensitivity of modern plates the head and body rest is only need for old people and nervous subjects, not DEAD people.