Hi
I can only talk from experiences down here at the South East, but a 40 call sign is usually used for a Cert II or IV which can be classed as a crew. Also the RTL might have been doing some driver training with a Cert II vollie. Most likely that the RTL was in Wallaroo providing some training and worked a shift for them today, hence the reason you have WL1. Again, only talking from experiences down here.
Will