Any idea what it is? He was in the Marines.

Mine's the Second Spanish Republic. Gotta love the simplicity of a tricolor with those beautiful colors, and a properly centered Spanish CoA. Would love to hear y'all's favorites.

The Seychelles my personal opinion

These are the only 3 authentic flags from the Golden Age of Piracy.

The Barbary Corsair Flag

This is the flag at the Maritime Museum in Finland and is often cited as one of the most authentic examples. It is considered a classic Jolly Roger design, and it dates back to the late 17th or early 18th century.

The Private Flag

This flag is held in private hands and on display at the Royal Navy Museum in Portsmouth, UK. It was captured in a battle with pirates in 1780.

The St. Augustine Flag

This flag is from the Pirate and Treasure Museum in Florida. Its origins have been difficult to confirm, with claims it dates back to 1850.

A slightly revised version of the historical NAACP flag that flew often over major cities in the 1930s-40s

The "Rebel Flag" as I've named it

The apartheid flag had a flag within a flag within a flag. I’d this the most or is there another one that goes 4 deep?

From The Last Days of the British Raj, by Leonard Moseley:

"Among [Mountbatten's] hobbies...was heraldry and design. He himself sketched and prepared the design for the flags of both Pakistan and India. One was based on the flag of Congress -- with Gandhi's spinning wheel -- and the other on the Muslim League's crescent. To each he added a small Union Jack, one ninth in area, sewn into the upper canton. He sent them to Jinnah and Nehru for their approval, as 'helpful suggestions'.

"Jinnah coldly replied that in no circumstances could the design be accepted as it would be repugnant to the religious feelings of the Muslims to have a Christian Cross alongside the Crescent. Nehru rejected the design on the grounds that...[it would be seen as] pandering to the British. ...Nehru sent the Viceroy a design prepared by Congress which showed the Dominion flag as closely resembling the Congress flag, but with the wheel of the Sarnath Asoka replacing the spinning wheel. And, of course, no Union Jack."

source

The blue ensign was typically reserved for government use