There was a big catch holding on to No. 755

It’s an instinct perhaps as old as baseball itself. When a ball is batted outside the field of play, whether fair or foul, fans give chase.

Richard Arndt was no different.

So when Hank Aaron of the Milwaukee Brewers belted his 755th home run, lining a slider from Dick Drago of the Angels into the left-field bleachers at Milwaukee County Stadium, Arndt took off running.

A part-time groundskeeper at the stadium and a longtime baseball fan, Arndt had been a diehard Milwaukee Braves loyalist growing up in Madison, Wis.

An Aaron home run ball would be a treasure, of course, but when Arndt reached this one a split-second ahead of a fast-closing pack of pursuers he had no way of knowing that he’d just taken possession of a rare baseball artifact.

It was July 20, 1976, and 2 1/2 months remained in the season. Surely the all-time home run leader would deposit another ball over another wall.

But Aaron, 42 and slowed by a knee injury, never did.

Arndt owned the keepsake of a lifetime: Hammerin’ Hank’s last home run ball.

“I just happened to be in the right place at the right time,” says Arndt, a social worker who has lived most of his adult life in Albuquerque but lived that one summer in Milwaukee. “It’s really been a blessing for our family.”

Arndt, 60, sold the ball for $650,000 to a Connecticut portfolio manager in 1999, but not before deflecting a number of curious attempts by others -- among them representatives of Aaron, he says -- to pry it away for less.

The father of two grown children says he will watch with interest as Barry Bonds of the San Francisco Giants closes in on Aaron’s record and offers this advice for the lucky fan who tracks down Bonds’ last home run ball: Be careful.

Adds Arndt: “And be sure to obtain good legal counsel.”

Arndt says that, for him, caution would not have been necessary if, on the night that changed his life, he would have been allowed to hand the ball back to Aaron, as he had hoped to do and as Atlanta Braves reliever Tom House did with Aaron’s 715th home run ball. Instead, when summoned to the dugout after the game, Arndt says he was met by an equipment manager who told him that Aaron was busy packing for the Brewers’ impending trip and wouldn’t have time to meet with him. Arndt says he declined an offer of a bat, an autographed ball and a photo of Aaron.

The next day, he was told he’d been fired for confiscating club property -- the ball -- and discovered that his final paycheck had been docked $5.

“I’m sure everybody would have handled this much more diplomatically had it been Sept. 30 instead of July 20,” he says, “but everybody assumed he would hit more home runs. Nobody thought that would be his last home run.”

Still, Arndt had the foresight to phone the Milwaukee Journal, which ran a short item on his firing, thus documenting that he was in possession of the ball. A few weeks later, he says, he approached Aaron after a game and asked the home run king to sign the ball, “Hank Aaron 755,” but Aaron refused.

At season’s end, Arndt says, an electronics company with an apparent endorsement tie to Aaron offered $1,000 and a television set in exchange for the ball. Arndt says he nearly accepted but reconsidered.

A few months later, after moving back to Albuquerque, he stored the ball in a safety-deposit box, where it remained for most of the next 22 years.

But he brought it to Phoenix in 1994, where Aaron signed it at a card show without realizing its significance, later saying that he’d been duped. Two years earlier, Aaron had told Sports Illustrated that he -- not Arndt -- was in possession of the 755th home run ball, a claim the former slugger later recanted.

At some point in the ‘90s, Arndt says he was contacted by an Aaron representative who told him that she believed the ball’s value had decreased over time because Aaron was unpopular, had not played in a major market, etc., etc.

On behalf of Aaron, Arndt says, the woman offered $5,000.

Finally, after Arndt turned down a $100,000 offer from a California collector and Mark McGwire and Sammy Sosa staged their baseball-reviving home run race of 1998, the former groundskeeper decided it was time to sell. A lawyer representing Aaron, he says, claimed that Aaron was entitled to half the proceeds, a proposition that Arndt surprisingly agreed to -- if Aaron helped market the sale.

Though Aaron never did, Arndt nevertheless agreed to donate 25% of the net proceeds -- more than $155,000 -- to Aaron’s Chasing the Dream Foundation, an Atlanta charity that benefits underprivileged children.

“He hit it,” Arndt says of the famous home run ball, “so I felt like he was entitled to something. I thought it was the right thing to do.”

As for the rest, a check made out to Arndt for nearly $462,000, “Uncle Sam got a good chunk, the state of New Mexico got a good chunk, I gave some to our church and my wife and I gave some money to our children,” says Arndt, who also invested part of the windfall. “We were able to do some good things with it.”

All because he followed his instinct and chased a souvenir.

“When I picked up that ball,” Arndt says, “I had no idea.”

jerome.crowe@latimes.com