In the old days, forty straps of notes would be placed between two wooden blocks, with a label like this on one end, held together with steel bands. The whole thing was then wrapped in brown paper, with a second copy of the label on the outside.
Sometime around 1990 (plus or minus a few years) the BEP switched to shrink-wrap for the bricks. They also added a level of packaging: they now wrap ten straps to a bundle and four bundles to a brick, instead of just forty straps into a brick directly. The Fed tends to break the bricks down into bundles before shipping them out, so you don't often see labels for anything larger than a 1000-note unit reaching the public any more.
[ Reference: "Numbers" on the CoinTalk Paper Money Forum ]
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