Its logical, there must have been a good understanding between the musician and the centurion - the musician would need to know what the officer wanted and what signal to play in order to relay the instructions to the soldiers - shouting orders in the middle of a battle would not be enough and could easily be misunderstood.
I tried a bit of graffiti on the back of one of the shields - a bit of Roman humour harking back to the good old days of Julius Ceasar - Veni Vidi Vici. I like the idea of Roman graffiti/humour, so need to research more Latin phrases - I looked at some surviving wall graffiti from Pompei but its all a bit too rude and certainly not contextual to the back of a soldiers shield, even though the soldiers sense of humour would be pretty rough.
In the Early Imperial Roman period a legionary was appointed as the medic - medicii. I guess it would continue into the later period so I have marked this mans rank on the back of his shield.