My guess is that the Templar letter of credit and money transfer system was based on passwords, which is how the Muslim hawala system of alternative banking works. Hawala developed in India in the Fifth Century. It became part of Muslim trade relations with medieval Italy, was widely adopted into European banking, and even prompted European development of the law of agency to recognize how hawala worked. Perhaps the Templars had a different system, but the lack of evidence for that suggests otherwise.
I have read variously that the Templars were the first international bankers. If not, they were among the earliest. Whatever their methods and codes, they appear to have worked. Their mistake was loaning to royalty, a mistake that perhaps Nicholas Flammel had some experience with.