No group has claimed responsibility, but the disappearance of the hubcap-sized brass symbol has stirred anger in Thailand, where elections are promised but power is firmly in the hands of the ruling generals and King Maha Vajiralongkorn.
The junta said it had no idea who had removed the plaque, which had been embedded in concrete at a square in Bangkok's leafy historic quarter. It is close to parliament, to a royal throne hall and to an army barracks. The area is also surveyed by several police posts.
The plaque went missing on Friday, police said, and was replaced by one celebrating the monarchy.
"Why should we take responsibility? If that is the case should we take responsibility for everything that disappears in this country?" Veerachon Sukhonthapatipark, a deputy government spokesman, told Reuters.
Police said they were investigating.
The removal of the plaque comes nearly three years after the army overthrew an elected government in the name of ending violent political turmoil it blamed on civilian politicians in the Southeast Asian country.
Although the junta has promised elections - now expected by the end of next year - critics say a new constitution is designed to ensure the military's influence over civilian politics for years to come.
"The plaque was a symbol of equality in Thailand," Thanawut Wichaidit, spokesman for the opposition United Front of Democracy Against Dictatorship group, told Reuters.
"Past governments never thought about taking the plaque out but this government, with its extraordinary powers that are above the law, has allowed this to happen which begs the question of when and whether Thailand will truly return to democracy."
A Change.org petition demanding an investigation into the missing plaque had attracted more than 2,400 signatures.
The plaque marked a spot where a crucial stage in the events of 1932 unfolded. The coup was staged by Western-influenced young civilian bureaucrats and army officers to end absolute monarchy and establish a parliamentary system.
Over subsequent decades, the army gained a central role and Thailand has experienced a repeated cycle of elections, coups and protests. The new constitution is the 20th since then.
Though a constitutional monarchy, the Thai crown is a powerful political force and late King Bhumibol Adulyadej sometimes intervened during times of crisis.
His son, King Maha Vajiralongkorn, who ascended the throne last year, has shown himself increasingly assertive - requesting changes to the new constitution that underlined his powers before signing it into law this month.
Rights groups say freedom of expression has sunk to new lows under royalist Prime Minister Prayuth Chan-ocha.
Several government critics have been jailed under strict laws against insulting the monarchy that carry a sentence of up to 15 years for each offence.
At least 74 people have been charged with lese majeste crimes since the military took power in 2014 according to iLaw, a Bangkok-based legal monitoring group. Half of them have been jailed.
For more than a decade, Thailand has seen colour-coded protests – mostly by conservative, royalist "yellow shirts" and their opponents, the red-shirted supporters of new populist political forces.
They see attempts by the military-led establishment to exert control over politics as a threat to Thailand's democracy, that can trace its roots back to 1932.
Not everyone was unhappy with the removal of the plaque.
"It was a reminder for Thailand to follow Western democracy ... which might not be suited to Thailand anyway," Nathadej Meksawat, a retired Thai army officer and political observer, wrote on his Facebook page.
For Sunai Phasuk, Thailand researcher for U.S.-based group Human Rights Watch, the removal of the plaque and the start of a fourth year under military rule marked a bleak chapter.
"The reaction to the disappearance of the plaque is very telling of the challenges and the tension between supporters of a return to pre-1932 Thailand and, on the other hand people who strive for democracy," said Sunai.