What was the method used by the man who made over 100 million yen by stealing coins from public telephones?

In the 1980s, there was a pay phone thief in the United States who made $1 million a year by stealing coins from pay phones. The thief traveled across state lines and used a unique unlocking device to carry out his thefts.

Pay-Phone Thief's Odyssey Touches Companies for $500,000 - Los Angeles Times
https://www.latimes.com/archives/la-xpm-1987-12-08-me-27444-story.html

Phone Companies Seek Coin Thief
https://www.oklahoman.com/story/news/1988/02/14/phone-companies-seek-coin-thief/62661920007/

The 'Pay Phone Bandit' Who Baffled the FBI in the '80s
https://www.mentalfloss.com/posts/pay-phone-bandit-baffled-fbi

The man who continued to steal coins from numerous public telephones was an Ohio native named James Clark. Clark was arrested in 1968 for counterfeiting money and served a three-year sentence. Ten years after his release, he began stealing public telephones in the 1980s.

Clark used his experience as a mechanic to create a locking device for pay phones. Different types of pay phones have different locking systems, but Clark focused on Bell-affiliated pay phones. The exact mechanism of the locking device has not been made public, but Clark continued to steal coins by using a specially made device to check the amount of coins in the pay box, removing the protective plate from the pay box with the locking device while acting engrossed in the phone, removing the pay box and replacing the plate.

At the time, public phones worked properly without a pay box, so the thief was not noticed until the attendant removed the plate to collect the money. It was difficult to catch Clark because he had traveled far away by the time the attendant came to collect the money. Clark mainly targeted pay phones along major highways, and continued his pay phone theft while traveling through 30 states. Clark used the money he stole for food and lodging, and when staying at hotels he used the name 'James Bell' after Bell.

At that time, public telephones only held about $150 (about 24,000 yen) in coins per unit, so there were few thefts targeting public telephones. However, Clark continued to steal coins from a large number of public telephones while traveling across states, and the annual damage amounted to about $1 million (about 160 million yen).

There have been reports of Clark being seen in various places, and most of them have the same characteristics: 'He was about 5 feet 9 inches (174 cm) tall, wore a baseball cap pulled down low, and wire-rimmed glasses. His ponytail was sticking out from the back of his hat, and his cowboy boots were visible from under his trousers.' Another notable feature was that he 'used multiple bundles of quarters to pay.'

The state and FBI investigations were proving difficult, but in 1985, an informant provided information that Clark should be investigated in more detail. Investigative authorities entered the trailer where Clark lived with a search warrant. Although Clark was not there, they found a 'payphone lock that was likely used for burglary practice.' A nationwide warrant for Clark's arrest was then issued, and Clark was arrested in California in August 1988.

Clark's crimes were nationwide, so the court process was very complicated. Clark first pleaded guilty in Ohio to only five counts of theft, and the total damage from the five thefts was only $500 (about 80,000 yen). Clark was sentenced to three years in prison in Ohio, and then served an additional three years in 1990. Clark passed away in 2012.