The originals were probably formed using a die set in a hydraulic double-action press: the first part of the very slow stroke clamps the entire sheet flat, the second part is the actual forming die that stretches the material to shape. The clamping before forming prevents the sheet from wrinkling around the edges and away from the forming die.
It always amazed me just how far the right material can be stretched - deep stainless steel wash-up sinks, for example, are/were done this way.
Amazing work, Colin. Very, very skilful.
Do you have an original .50" Vickers ammunition box to use as a sample?
Mike