The shield was originally a prop from a costume store, taken by Guardian (Jim Harper) back in the 1940s. He used it throughout his career.
A similar shield would later be used by his clone replacement Jim Harper II.
The shield was virtually invulnerable to nearly all forms of physical attack, but a sufficient amount of applied pressure could crack or even shatter the shield. During the "Battle for Metropolis", The Underworlder known as Clawster fought against the Guardian and broke the shield in half.  The shield has since been rebuilt.
The shield has been upgraded to include anti-gravity properties that enables Guardian to use it as a hover-craft. 
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