"it grew out of in-group reference to the names of government boards and bureaus."

More likely from the stencil marks on sides of cases.

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A medieval knight in full fighting armour, afoften ter his years of training as a squire from a young lad, could leap onto his saddle from the ground without using the stirrups. Those huge suits of thick plate armour from a much later period, ARE as heavy as all shit, and were used for only tournament jousting, and the rider was usually lifted into the saddle by means of an A-frame style of crane.