Would it have something to do with camoflaguing the "addition" to the turret which took the recoil of the 17pdr, made the Firefly turret very distinctive.
(I cant imaging anyone wanting their Sherman to look like a Firefly, picked on first time)

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Whe're off to see the Wizard, the wonderful wizard of OZ - 6 Australian Infantry Div, Bardia January 1941