In the World War II most Australian army personnel were allocated a service number which denoted their state of origin; N for NSW, V for Victoria, etcetera. Enlistments relating to the Second AIF (from WW II) were followed with an X. Also during World War II, Old Melbourne Gaol was drafted into service to hold military prisoners, most of whom had gone absent without leave.

Given the "VX" prefix on this scratching, I think it's safe to say that this cell once held a Victorian member of the AIF who was doing a bit of time for absenteeism.

These days of course he'd probably just update his status on Facebook rather than scratching his service number into the wall.
-----
Edit August 2024: I wrote that last paragraph when Faceplant was at its zenith and seemed unstoppable. It gives me hope for the future that younger generations in particular are starting to turn on it as being uncool and antiquated given that I regard social media in general and Faceplant in particular as being one of the most destructive social forces in the last 20 years. Unfortunately a certain (personal opinion only) android sociopath who runs it will never be poor, but if this prison cell were ever to go back into service I'd love to see Suckerberg locked in it for 23 hours per day.