The lower (LEL) and upper (UEL) explosive limit data quoted in the pure gas PDF files is taken from:
AS/NZS 60079.20:2000 Electrical apparatus for explosive gas atmospheres - Data for flammable gases and vapours, relating to the use of electrical apparatus.
Note: that there are differences between this data and that published in overseas sources, for example the Compressed Gas Association (CGA) in the USA.
The ‘Ref. No’ quoted in the PDF documents refers to the Ref. Column in Table A.1 – Flammability Data in this Standard.