Hi Mike,
I started working for the army in 1974 and I think it was 1975, a QL with Office body came in for a gearbox change. I believe the truck was cast (disposed of) the following year. Most remaining QL's in units would have gone around that time. It was interesting to see documents of vehicles still in service around that time, with a number of WW2 veterans hanging on. Not sure of exactly when but think it was late 70's a number of WW2 Leyland Hippos were brought out of storage from a British vehicle depot in Belgium to supplement the heavy vehicle fleet until the Foden 16 tonne came into service. We had a number in the area.
Going back to the QL in question, I think it could have been kept in service if it had a specialised body or role, and as the Key Card has "Binned" against its entry, then this would be why. The same happened with other trucks that had special roles and bodies. The earlier disposal dates you have noted would be for normal GS cargo trucks.
My own QL had been transferred to the Danish army in the late 40's and disposed of in 1987 from reserve storage.
cheers Richard