interesting friend, and referring to the symbol of the oak so it is, denotes strength and firmness, confidence, protection and others, it is even used, for example in ancient Greece and its mythology, the oak was a sacred tree for Zeus, as it was taken into account as a symbol of justice and moral rectitude, It is also said that the club of Hercules was made of oak wood, if the symbolism is linked to ancient myths and if I'm not wrong is symbolized in the currency that way to refer to the monetary unit of the united kingdom, in fact also formerly to abbreviate was denoted with a (d) that referred to the denarius.