Government Code & Cypher School (and a hidden message)

During World War I the British Army and Royal Navy both had separate intelligence agencies. Military Intelligence 1b, MI1(b) and Naval Intelligence Department 25, NID25, respectively. NID25 was initially known as Room 40, or 40 O.B., with O.B. referring to 'old building'. 

Following the conclusion of World War I the Cabinet's Secret Service Committee recommended that a peacetime codebreaking agency should be created. Lord Curzon, chairman of the Cabinet's Secret Service Committee, tasked Hugh Sinclair, Director of Naval Intelligence, with creating the new department.

Hugh Sinclair merged staff from NID25 with MI1(b), with the new agency consisting of between 25-30 officers, with roughly the same amount of clerical staff. Victor Forbes, of the Foreign Office, chose the cover-name 'Government Code & Cypher School (GC&CS).

The Government Code & Cypher School moved into Watergate House, Adelphi, with the Admiralty's Alastair Denniston, previously an NID25 officer, as its operational head. 

Its public directive was "to advise as to the security of codes and cyphers used by all Government departments and to assist in their provision". However, it also had a secret directive: "study the methods of cypher communications used by foreign powers".

Although officially formed on November 1, 1919, the Government Code & Cypher School produced its first decryption on October 19, 1919.

In 1921 the offices moved to the Broadway Buildings, opposite St James's Park, in the same building as the SIS, and came under the supervision of Hugh Sinclair. At the time Hugh Sinclair was Chief of SIS and Director of GC&CS.

On February 14, 2019, HM Queen Elizabeth II unveiled a City of Westminster plaque to mark the centenary of GCHQ.

However, keeping with the codebreaking, communications and cypher work of GCHQ, the plaque contains a hidden message, which most people don't even realise. 

Can you uncover the hidden message?

If you can't figure it out, there is a clue below. 

Various characters, on the plaque, have a dot or dash beneath them. 

If you find them all it reveals the message... 

Highlight the space between the quotation marks, below, to uncover the answer.

"1 HUNDRED YEARS"

So, did you work it out, or did you cheat?