In August 1946, a Maryland man went to his usual lunch spot for a bite to eat. Finding the diner full, he asked a man sitting alone if he could share his table.

The man, as it supposedly turned out, was Adolf Hitler.

The dictator was dressed “cheaply” and “very nervous,” according to the witness. Hitler fidgeted and played with his napkin. His lunchmate called the authorities.

In 1948, a letter arrived at a Spanish-language newspaper. The writer claimed he’d last seen Hitler in Bogota, Colombia, 10 days previously and that he was “in perfect health.” The Fuehrer had plans to conquer the moon and Mars.

Conspiracy theories regarding the death of Hitler are legion. Tales about his escape from a German bunker began cropping up just a few hours after his supposed death on April 30, 1945.

Now a new book claims to put the kibosh on all the rumors and definitively solve the mystery of what happened to the Nazi monster.

“The Death of Hitler: The Final Word” (Da Capo Press) is written by French journalist Jean-Christophe Brisard and Russian documentary maker Lana Parshina.

The duo ventured to Moscow to comb historical documents stored in secret archives. They were also allowed to examine a skull fragment containing a bullet hole and teeth supposedly taken from the German leader’s body stored at the library belonging to Russia’s notorious Federal Security Services.

The authors spent months negotiating for access to the depositories presided over by stern former-Soviet librarians and humorless guards wearing severe black uniforms.

“How foreigners — even Lana with her perfect Russian and her old Russian passport — like us could get official access, that was the biggest challenge,” Brisard tells The Post.

Among the 7 million documents stored in the Russian archives, the authors found top-secret accounts detailing the interrogations of members of Hitler’s inner circle, who were captured when Soviet troops took Berlin.

What emerged was a detailed, if sometimes frustratingly contradictory, chronicle of Hitler’s final days.

By April 1945, the Fuehrer had retreated to a heavily fortified underground bunker near the Reich Chancellery at 77 Wilhelmstrasse. On April 29, with Russian troops just blocks away, Hitler married girlfriend Eva Braun in a 10-minute ceremony. Just before the union, Hitler dictated his will to his private secretary. The document stated that he and Braun had chosen death and that their bodies should be burned upon discovery.

Hitler tested cyanide on his dog, Blondi, preparing for the end.

The next afternoon, he emerged from his room and quietly said goodbye to his closest aides. What happened next is unclear.

Hitler’s valet, Heinz Linge, was stationed outside the door and told the Russians he heard a gunshot. He ran to alert Hitler’s secretary Martin Bormann, and the two entered the room to find their leader and his wife dead.

The bodies were wrapped in blankets, carried upstairs, doused with gasoline and burned.

The Russian files reveal that the Soviets collected numerous accounts (often under extreme duress) of the events from other bunker dwellers, and the interrogators grew irritated by the disparities, believing they were being lied to.

Some Nazis reported hearing a shot, others didn’t. Some reported seeing a bullet wound in Hitler’s head, others didn’t. Had Hitler shot himself in the mouth or the temple? Or had he not shot himself at all and instead taken cyanide?

The Soviets preferred the latter narrative, because they believed it showed cowardice on Hitler’s part.

A 1945 Russian autopsy performed on the burned corpses recovered near the entrance to the bunker backed up the cyanide hypothesis. A crushed glass capsule was found in the man’s mouth and the smell of bitter almond was strong, indicating the poison.

The bodies were then buried, although they were later exhumed and cremated. But fragments of the jawbone were saved.

The skull piece was discovered by the Soviets in 1946, near where the burned bodies were found. It was stored in an archival safe, uncataloged until stumbled upon by a librarian in 1975.

To examine this physical evidence, the authors brought in Philippe Charlier, a French forensic pathologist who was nicknamed “the Indiana Jones of graveyards” for his role in identifying high-profile historical bodies.

Charlier was acceptable to the Russians, not just because of his reputation, but because he was not American. Anything but an American.

The cautious Russians allowed only a visual analysis of the skull piece, and Charlier was able to determine that it belonged to an adult male and that it had been burned. But was it Hitler’s? The doctor couldn’t say.

Hitler’s teeth provided more clarity. Charlier was able to compare the jawbone with X-rays taken of Hitler in 1944. His conclusion? It was a match. No doubt. There was “perfect agreement.”

Hitler did not escape Berlin in 1945. He died in that bunker.

But the how of Hitler’s demise remained a mystery.

Charlier was also allowed to analyze the teeth with a microscope, and discovered strange blue stains. Charlier was stumped. Could cyanide have caused the discoloration?

One of the bigger breakthroughs came completely by accident. Once Charlier had returned to France after the initial examination, he discovered some nearly microscopic pieces of dental tartar from Hitler’s teeth stuck to the rubber gloves he wore for the examination.

He examined these grains by electron microscope and found more evidence strengthening the case. Within the sample, he discovered vegetable fibers but no meat, indicating the teeth belonged to a vegetarian, as Hitler was.

The tartar also was scanned for traces of three metallic elements that would indicate that Hitler had shot himself in the mouth, as his chauffeur, Erich Kempka, had told the British in 1945.

No traces were found, meaning Hitler almost certainly shot himself in the temple, as some witnesses stated.

As for the cyanide? Charlier was unable to come to any conclusion and is still unsure what caused the blue stains on the teeth.

Without a more thorough examination, they will remain a mystery.

And Russia apparently isn’t exactly welcoming any new inquiries. The teeth — among the most fascinating historical artifacts of the 20th century — were returned to the repurposed cigarillo box in which they’ve been casually stored, wheeled deep into the archives and dropped onto a shelf alongside God knows what other relics. It is unclear when, or if, they will ever be seen again.

This story originally appeared in the New York Post.