A 1928 armored Cadillac V-8 believed with near-complete certainty to have belonged to Al Capone will be auctioned off in Plymouth Michigan on July 28.

It has long been generally accepted that the car did indeed belong to the legendary gangster; and in 1932 - the year after he went to prison for tax evasion and bootlegging - it was purchased for use in a traveling carnival. In the ensuing decades, it has been part of museum collections on both sides of the Atlantic.

Much of the car's armor plating was (inexplicably, from a collector's standpoint) removed during a restoration in the 1950s, but the bulletproof glass and a dropdown rear window remain. The window was supposedly installed so that Capone's men could fire on pursuers in the event of a car chase.

According to RM Auctions, which will be conducting the sale, Richard "Cappy" Capstram, then 93, was recorded giving his recollection of Capone's men coming into his father's auto body shop in the summer of 1928 to order the work done.

"My dad said, 'We don't do that kind of work here.' And they said, 'You do now,'" Capstran recalled.

He also remembered that Capone showed up personally to pay for the work, and gave his father double the asking price. When Capone heard that the ten-year-old had assisted his father, he gave him a $10 bill.

In addition to the dropdown window and the bulletproof glass, when the car left the Capstran shop, it had nearly 3,000 pounds of armor plating, and modified windows. These could be raised an extra inch or so to reveal circular cutouts large enough for the muzzle of a machine gun.

RM is offering interested buyers a look at the documentation linking the car to Capone. The auction house says, "With provenance now known since 1932 and strong supporting information, this example is not only a silent witness to the bloodiest era in American organized crime, it also represents the development of the modern armored sedan."

The car is to be auctioned by the estate of John O'Quinn, the well-known Texas trial lawyer and car collector who died in a road accident in 2009. The asking price is $300,000-$500,000, but another car recently sold by the estate was the subject of a bidding war that doubled the price. That car, a 1912 Rolls-Royce, ended up selling in England for over $7 million.