Alan Squindo of RM Auctions told The Detroit News the car could go for as much as $500,000 at the sale in Plymouth, Mich. The event was not open to the public.
Squindo said the car would be valuable even without its history.
"This is a very special automobile, not withstanding its Capone history," Squindo said. "It is a luxury car from a classic era as one of the first bulletproof cars that still survives. It's a very heavy car and a very good looking car."
Richard "Cappy" Capstran, 93, told the Chicago Tribune he helped his father install the armor plate. In 1928, Capstran was 10 years old and spending the summer working in his dad's body shop in Chicago, a few blocks from Capone's base at the Metropole Hotel.
Capstran, who now lives in Wisconsin, told the Tribune in a phone interview, his father had straightened a bent fender on Capone's Buick.
"We fixed the Buick -- did a nice job -- then three or four days later, they brought in the Cadillac," Capstran said. "Dad said 'You want us to stripe it?' And they said 'No, we want you to cut out the back and install armor plate.' My dad said 'We don't do that kind of work.' Capone's men said, 'You do now.'"
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