King Norodom Sihanouk ascended to Cambodia’s throne in April 1941 at age 18, almost exactly five years before King Bhumibol Adulyadej ascended to Thailand’s in June 1946, also at the age of 18.

Both were fluent French and English speakers and shared a deep appreciation for jazz, making names for themselves as saxophonists. Sihanouk died in October 2012 aged 89 and – almost four years to the day – Bhumibol followed last week, at the age of 88.

As towering figures in the modern histories of their neighbouring Southeast Asian countries, the two kings’ lives and interests intersected like those of few others through history – but that did not mean the two were always so close to each other.

“It might have been expected that Sihanouk and Bhumibol, both French-speakers of similar age and predicament, would become soul mates,” Paul Handley noted in his 2006 The King Never Smiles: A Biography of Thailand’s Bhumibol Adulyadej.

Instead, Handley noted, Bhumibol came to consider his Cambodian counterpart “a nuisance, in part because in 1954 Sihanouk apparently borrowed a gold-plated saxophone of the king’s and didn’t return it”.

Little else has been written about their six-decade relationship – during which Sihanouk abdicated, was overthrown as a civilian leader, led an armed resistance and was then crowned king once more, and it is one of the few windows into their time together.

Yet the claims Sihanouk took the saxophone were always disputed by the man himself, said Julio Jeldres, a research fellow at Monash University’s Asia Institute in Australia, who was Sihanouk’s official biographer before the king’s death four years ago.

“I have seen no record of King Sihanouk meeting King Bhumibol on this occasion, but it appears that the Thai King in a friendly gesture loaned to him a gold-plated saxophone, which Sihanouk could use while in Bangkok,” Jeldres said in an email.

“Later on, the former US Ambassador to Thailand, Alexis U. Johnson, wrote that Sihanouk had taken the gold-plated saxophone with him,” he said.

Jeldres said he believed the incident in fact occurred after an ill-fated 1953 trip, and not during 1954 as noted by Handley, and that Sihanouk always maintained he left the saxophone behind in his hotel room in a huff after failing to secure Thai support for Cambodia’s drive for independence from France.

“The late King Father was very offended by this story and told me in 1988 that the saxophone had been left behind at the hotel in Bangkok after he suddenly decided to leave because of the cold reception he had received and the lack of Thai support.”

It’s an explanation accepted by Prince Sisowath Thomico, a nephew and adopted son of Sihanouk, who said he did not believe the king would have taken something that was not his – or done anything else to harm relations with another king.

“I am very suspicious about that. It was not in King Sihanouk’s personality not to return something, and he always kept very courteous relations with King Bhumibol,” Thomico said, noting that Sihanouk had once demoted a minister for criticising Bhumibol. “King Sihanouk always expected the royal government to respect a king – even though we did not have very good relations with the Thais then.”

Yet it was not only the disputed fate of the golden saxophone that made the 1953 trip so important for the future of relations of Sihanouk and Bhumibol – there were also the circumstances of the trip. In fact, Sihanouk had decided to pay a visit to Thailand on a whim, without giving notification, while driving near the Cambodian-Thai border, according to French historian and author Henri Locard.

While in a convoy between Siem Reap and Battambang, explained Locard, Sihanouk simply announced that he wished to cross at the Poipet-Aranyaprathet checkpoint to seek support from the Thais for his struggle for independence.

“When in Svay Sisophon, the King suddenly decided to split the procession: Penn Nouth, the nominal prime minister and followers were asked to continue to Battambang, while Sihanouk and his court proceeded straight west to Poipet,” Locard said.

“Neither the Cambodian delegation nor – worse – the Thais themselves had been forewarned of the new plans,” he continued. “He came totally uninvited and the Thais gave him a cold reception. They refused to support his campaign for independence.”

Jeldres said an official state visit the next year – after Cambodia gained its independence – fared better, with Sihanouk being “received at the airport by King Bhumibol and given a warm welcome” and also stressing to the Thais that he bore no grudges.

However, it was the only official state visit Sihanouk would ever pay to Thailand as king, and in the 1980s – when he led the coalition of Thai-backed resistance forces fighting Prime Minister Hun Sen’s regime – he would again come to have complaints.

“During the 1980s, he complained that every time he visited Thailand as President of the Cambodian Coalition Government, he would be made to wait outside King Bhumibol’s office for half an hour or so before seeing the Thai monarch,” Jeldres said. “Once he became King again in 1993, he paid State visits to all of Cambodia’s neighbours, including Malaysia and Singapore, but excluding Thailand.

“Lastly, many Cambodians were deeply disappointed that the Thai Royal Family did not send any representative to the funeral of the late King Father, as the Japanese and Lao Royal Families did,” he said.

Ultimately, though, according to Thomico, the relationship between the two kings – even with their similar backgrounds – could only ever have been constrained by their positions, as their nations followed vastly different paths last century.

“The word ‘friend’ could not be used between the two of them, because their relations were filled with protocol, but there was mutual respect,” he said. “And I never heard him criticise King Bhumibol.”