It's all related to the symbols and graphics used to display hazard warnings related to explosives. Generally seen on storage containers/buildings and trucks transporting any type of explosive materials be it civilian stuff all the way through small arms ammunition up to aircraft bombs and plastics. It's all very inter-linked with the air force armament trade, we tend to get lots of custom apparel, coins, cufflinks etc done up using the same basic symbolism.
Within the military it'll generally be labels like these all over everything we work with or on:
I picked 1.1 for the engravings because that's the most dangerous stuff with the most cool factor. Stuff like P7, 500-2000lb bombs, air-to-air missiles, claymores etc would fall under the 1.1 category. The larger numbers represent lesser hazards, so 1.3 for example is just a fire hazard rather than all out explosion, things like smoke grenades and large flares.