An United Nations ADR plate displaying a notice for gasoline (petrol), UN Number 1203.
<p>The 33 is a 'Hazard Identification Number' meaning "Highly Flammable Liquid, flashpoint below 23 °C (73 °F)." 
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<a rel="nofollow" class="external text" href="https://d18hkfaesybon0.cloudfront.net/ncec/media/assets/downloads/ncec-emergency-action-code-list-2017.pdf">Source, PDF Page 28.</a>
Drawn by <a href="//commons.wikimedia.org/wiki/User:The_Navigators" title="User:The Navigators">The Navigators</a>.
2018-11-04
<a rel="nofollow" class="external text" href="http://www.unece.org/fileadmin/DAM/trans/danger/publi/adr/adr2007/English/05-0%20E_Part%205.pdf">United Nations Economic Commission for Europe ADR Guidelines</a>, PDF page 26, document page 254.