The letter (if one) designates the type of ship. X = Super destroyer. T = torpillier (destroyer)
 
The first number is the number of the flotilla it belongs too. The second number is the position it holds in the flotilla.
 
If there are three numbers, the first two are the flotilla number and the last is the position.
 
Red numbers were preferred on light grey ships and white-edged black on dark grey ships.
 
The decals are specifically sized to GHQ French castings but probably will work on other manufacture castings