This gentleman, wearing his best shoplifting trousers, is holding a bicycle that is ‘4 or 5 kilograms lighter’ through its use of groovy new Duralumin (often called Dural in the UK). Duralumin was an alloy of aluminium with small amounts of copper (and possibly manganese and magnesium) and could be age hardened with a simple process of heat treatment. It was first marketed in 1909, was a key component of the Zeppelins and became important in the aircraft industry during the early 1930s. As with so much aircraft technology it then rapidly found its way into the bicycle world.
In an age when bicycle salesmen blithely talk about 6000 and 7000 series Aluminium (usually without the even the faintest idea of what they are talking about), it is interesting to note that Duralumin was renamed as 2000 series aluminium. 2000 series aluminium can be hardened to have exceptionally high tensile strength, however it is suffers from stress corrosion - and anyone who has worked with pre-1960s aluminium components knows that they can suddenly break, usually for this reason. 7000 series aluminium is the nearest modern equivalent - and is much more resistant to stress corrosion.
(Source gallica.bnf.fr/ Bibliothèque nationale de France)