What is the meaning of the '2' on some Compressed Gas Hazard signs in the lab? The main chemistry prep room at my institution (and also at the ambulance station) is labelled with the standard green compressed gas sign.  However, there is a small '2' included within the diamond.  None of the chemistry technicians, tutors or ambulance staff seem to know what it is referring to and I can't find any reference of it on the internet.  Would anyone here happen to know?



The related products of this online provider show that other hazards have different numbers but some also have two (e.g. flammable).  Is there a standard that is being referred to here?
The 2 refers to DOT (Department of Transportation) hazard class 2. This reference describes the different DOT hazard classes. 

As summarized in the comments:


  Class 2 (compressed gas) is divided into three subclasses (2.1 - flammable, 2.2 - non-flammable, non-toxic and 2.3 - toxic). I imagine the diamonds for Class 2 don't contain the subclass number because there is a different diamond colour/symbology for each subclass (as opposed to for instance 5.1 (oxidising agent) and 5.2 (organic peroxide), which have the same symbol and colours).