There is no entry in the Palmer roll for a "Tpr W.R Osbourne"; either killed, wounded or prisoner. So it appears to have been a "Served" acknowledgement on the plaque. As his rank is shown as "Trooper", I had a look on my IY CD and there are two possible candidates:
"23550 Tpr,.W. Osbourne, 76th Coy., 22nd IY.
"35179 Tpr. W.Osbourne, 74th Coy., 8th IY".
Of course, W.R.Osbourne could have served with the Cavalry - a few prospectives there. However, they are listed as "Private".