Might not saluting be also associated with "touching the forelock" - which was a sign of deference on behalf of someone of a lower status - to one of a superior status? ( e.g peasant to a Lord), many years ago.:salute::
I was watching a film of "Mutiny on the Bounty" a while ago - and was struck by the sailors' knuckling their foreheds in salute to the Captain - which looked very like that form of acknowledgement and deference to a superior rank.
Might also explain why a lower rank normally salutes the higher ranked one first - before getting his/her salute returned. It is thus a sign of acknowledgement and deference to another. The military has perhaps just "smartened the process up" - by making it sharper and more formalised? jb:salute::