According to the official army lists published by the War Office: officers who received the Waterloo Medal had a "W" prefixed before their name in the lists.
Alexander Cavalie Mercer has a "W" before his name in the army lists. The earliest I saw was 1824.
This is a no preview work: might refer to Mercer's medal.
The Numismatic Circular
By Spink & Son
Item notes: v. 27-29
"VF 25 Waterloo. 181 5. Captain Mercer's Medal of the Royal Horse Artillery. ...
The second son of General Mercer R . E. was born in 1783. ..."