A priceless painting. An English lord. An aristocratic ancestor looking for some easy cash. That, friends, is why the Viscount and Vicountess Allendale showed up at the National Gallery of Art Tuesday.

Wentworth Peter Ismay Beaumont and his wife Tessa the made their first-ever trip to the gallery’s West Wing to see the “Adoration of the Shepherds.” Giorgione’s famous 1505 painting is also known as the “Allendale Nativity”: the viscount’s great-great-great-grandfather bought it in 1847 and it was the centerpiece of his family’s private collection for almost a century. In a real-life version of “Downton Abbey,” his grandfather sold the painting in 1937 to a British art dealer for $315,000 — about $5 million in today’s dollars. Its value today? Several hundred million.

“I suspect he wanted some cash,” said the viscount. “They got a wonderful price at the time.”

Call it the one that got away:The painting is so valuable because it’s so rare: Giorgione, one of the greatest artists of the Italian Renaissance, produced only 15-20 works before he died in 1510. The famous nativity was snapped up for $400,000 in 1938 by American dimestore magnate Samuel Kress, who donated it to the National Gallery after displaying it his Fifth Avenue store department store during the Christmas shopping season. It even inspired the 2004 play “The Old Masters” which revisited the decades-old debate if the painting was really by Giorgione or his contemporary Titian — which would make it much less valuable.

The viscount’s father told his son of seeing the painting hanging in the family’s London home. “It was in the dining room, I think,” he said. “The king and queen [the future George VI] lived next door when they were the Duke and Duchess of York.” But he and his wife had never seen the masterpiece in person, which was the primary reason for the trip to the nation’s capital. VIP treatment, natch: David Brown, curator of Italian paintings, personally pointed out the finer points of the painting’s composition and influence.

“It looks absolutely great here, I have to say” conceded the viscount. “It’s a great deal more beautiful in the flesh.”

“In their home, it would be a valuable possession that the family could admire,” said Brown. “Here, we have five million visitors a year. No one comes to the National Gallery without looking at this painting.”

Don’t feel too bad for the family: They’ve still got a Rembrandt and other great paintings hanging in Bywell Hall, their 18th-century country estate in Northern England — and yes, it’s open for tours. Selling any more art? “No, we won’t be,” the viscount said with broad smile.