(CNN) At first glance they appear to be ordinary planks of wood marked with random scratches.

But archeologists say they're some of the oldest handwritten documents ever found in Britain -- and they include the first known reference to "London."

More than 400 ancient writing tablets were discovered during the excavation of a London building site, with the findings published Wednesday by the Museum of London Archeology.

The ancient Roman writing tablet bears the inscription "Londinio Mogontio" -- the earliest reference to London.

The central London site will soon be home to Bloomberg's new European headquarters, but the construction phase has revealed a treasure trove of Roman relics.

Along with hundreds of writing tablets, archaeologists have also found evidence of more than 50 Roman buildings and 15,000 other artifacts at the site.

Documenting Roman London

The tablets include a financial document dating back to January 8, 57 A.D. -- the earliest handwritten text ever found in the UK.

Another tablet features the address "Londinio Mogontio," meaning "In London, to Mogontius." Dated between 65 A.D. and 80 A.D., it is believed to be the earliest ever reference to London.

Among the tablets are the names of almost 100 people -- from slaves to judges and even brewers -- giving historians an insight into the day-to-day workings of Roman London.

Before now, only 19 legible Roman writing tablets had been found in London, making this discovery -- recovered between 2012 and 2014 -- all the more remarkable.

A rare find

The writing ledgers were in the shape of thin tiles, with a sunken area in the wood containing beeswax for people to write on.

Over thousands of years the wax has disintegrated, but the wood remains -- and on it, the indentation of people's handwriting.

Excavating the site where the writing tablets were found.

Luckily, the wood was encased in wet mud from the then-Walbrook river, which prevented oxygen from eroding the tablets.

As tablets were often reused, numerous layers of writing could be seen indented in the wood -- making the task of decoding the text particularly challenging.

Ancient codebreakers

Dr. Roger Tomlin, a lecturer in Roman history at Oxford University and an expert in Latin cursive writing, used a mixture of photography, raking light technology and microscopic analysis to decipher the text.

"I am so lucky to be the first person to read them again after more than 19 centuries, and to imagine what these people were like, who founded the new city of London," he said.

"What a privilege to eavesdrop on them: when I decipher their handwriting, I think of my own heroes, the wartime academics who worked at Bletchley Park," he adding, referencing Britain's World War II codebreakers.

Dr Roger Tomlin deciphers a writing tablet.

Thousands of years after the Romans helped lay the foundations for what is now the City of London, historians are still unlocking their secrets.

More than 700 artifacts from the site, including Britain's oldest writing tablet, will go on display at the new Bloomberg building in 2017.