If you select the type 4 (TT31 transponder) it works as real TT31, showing "FL---" in the On and SBY modes, see the demo slideshow here:
http://simvim.com/svc_custom_radio.html
When mode is in ALT or GND you will see the FL025 etc.
For types 1,3 the "FL" is not showing, only the digits are.
Vlad