The life of the notorious mobster Al Capone is under scrutiny more than seven decades after his death with an online auction in California of some of his most personal effects.

The three living granddaughters of the Prohibition era gangster known as "Scarface" are selling a collection of artwork, letters, a diamond encrusted pocket watch and even Capone’s personal Colt .45 pistol among a 174-lot inventory valued at up to $715,000 (£520,810).

The auction, which will be hosted live by Witherell’s of Sacramento on 8 October, is entitled "A Century of Notoriety".  It opens a window into the family life and private side of the iconic mafioso believed to have masterminded the 1929 St Valentine's Day massacre and scores of other mob hits during his brief but bloody spell as head of the ruthless gang called the Chicago Outfit.

One of the most fascinating lots is a  three-page letter to his son, Sonny, that Capone wrote in October 1931 from his cell in Alcatraz, the same month he was convicted and sentenced to 11 years in prison on federal tax evasion charges.

The personal letter Al Capone sent Sonny Capone, with a starting price of $12,500.

“To My Dear Son, Well Son of my heart, here is dear father, who loves you with all my heart and proud to have a son, as smart as you are,” Capone wrote, in pencil, in the letter that is expected to sell for between $25,000 and $30,000.

Bidding for the gangster’s Patek Philippe pocket watch, monogrammed and embedded with 90 diamonds, will start at $12,500. Other lots include a large number of photographs of Capone and family members, crystal glassware, figurines and other decorative items from the mobster’s homes in Chicago and Miami, earrings, cufflinks and money clips and assorted other jewelry.

Capone’s pistol, which has a $50,000 starting price

Meanwhile, Capone’s personal Colt .45 pistol, which he carried for protection, but which is not believed to have been used in any crime, is valued at up to $60,000.

Capone was released from prison in 1939 in poor health, and retired to his mansion in Palm Island, Miami, where he died in January 1947.

The collection belongs to three daughters of Sonny Capone, one of whom, author Diane Patricia Capone, 77, will appear at the preview event one day before the auction.

“What people don’t know is his personal story as a father and grandfather and his painful path of redemption while at Alcatraz,” she said in a statement issued by the auction house.

“That is the unknown Capone I talk about in my book, and it’s the story that comes to life with these family treasures.”

Diane Capone, who was born in Miami and helped nurse her grandfather in his final years and her sister Barbara decided in January to sell the collection.

“We were getting older and we were concerned that if anything happened to either of us, that people wouldn’t know what was what and what was the story that went with each thing,” she said.

A vintage silver print of Capone and associates at Hot Springs,

“Most of our treasures are things we hold in our hearts. We came from an extremely loving family, and that’s the greatest treasure that we have.”

Prior auctions of Capone memorabilia have raised significant sums, for example another pocket watch once owned by the gangster sold in 2017 for more than $84,000.