My guess is that it was a CFS machine at the time, but loaned to the ES, hence the addition of their badge.
The mystery I still haven't quite solved is why my early memories of Varsities at Strubby involve aircraft with the dayglow orange cheatline (with a thin black outline) that used to run along the fuselage and then zig-zag down towards the nose. All the photographes I've seen of CAW machines suggest they didn't carry this paint scheme but had a simple blue cheatline.
Anybody recall if any other non-CAW Varsities operated from Strubby at any stage? It would explain the mystery!