G 'day David.
The RFSU are organized in effectively the same fashion, only the names for formations differ.
51 FNQR uses the standard infantry company/platoon designations. Norforce and Pilbarra use the SASR squadron/troops , both have patrols of six men not sections of 9, as the smallest sub-unit.
For 51, our callsign (and by extension, vehicle designation) was company first (A,B,C,D/ Darwin, Kimberly, Centre, Arnhem- 1,2,3,4 respectively), then platoon (1-5) followed by Patrol designation (A-D).
Therefore, 11 belongs to alpha company/ squadron, 1 platoon/ troop. This differs from the 'real' army in that we didn't us the callsigns to designate seniority.
Now also remember that you need 2 vehicles per patrol, making you could find 2X 12A's out there.
I've seen a few marked 11, so maybe they stopped short of IDing the patrols and left it at platoon/ troop level.
I hope this helps.