During that summer, Kottke had a somewhat tedious job. He was to sit in a garage and put together Apple I computers, the devices his old college friend and his co-founder Steve Wozniak had designed and built. Now, nearly 40 years later, one of those Apple I computer boards that Kottke helped assemble is up for auction—and it could be worth as much as $1 million, according to a listing on auction site CharityBuzz.

“CharityBuzz is thrilled to be auctioning the most unique and quite possibly the first Apple I computer in existence,” the company’s CEO Coppy Holzman tells Fortune.

Apple I was released by Apple (AAPL) in 1976. The computers were designed by Wozniak—or Woz as he’s come to be known—and partially hand-built by some of the company’s earliest employees. Jobs was the salesperson in the operation, going off to stores to see if he could sell the computer that Wozniak had designed.

Finally in 1976, Jobs had a taker: the Byte Shop, a computer store in Mountain View, Calif. that tailored its offerings to computer enthusiasts hoping to create and use a wide range of programs, including word processors and more.

However, Jobs was met with a bear of a challenge: to get 50 Apple I computers to the store within 30 days. Jobs made good on that promise by relying in part on his workers and in part by using a manufacturing company that was able to put some of the board together before Apple’s team did the rest of the work.

It was the start of what would become Apple.

Now, though, there are only 60 to 70 Apple I computers in existence, according to Corey Cohen, an Apple historian and Apple I expert who spoke with Fortune in an interview. He’s among the few people in the world who can spot differences between a replica and the real thing.

Over the years, Apple I computers have been traded near and far by those who’ve wanted to create applications for the old-time computer, own a piece of history, or simply make some extra cash on the hardware. For quite some time—especially when Apple was in serious trouble in the 1990s—they were surprisingly cheap.

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Over the last decade, though, everything has changed.

After Apple’s resurgence at the hands of Steve Jobs, Apple I prices started to creep up, Cohen says. After Jobs died in 2011 after a long battle with cancer, prices soared even higher.

In 2014, The Henry Ford Museum paid top dollar for an Apple I computer: $905,000. Now, on August 25 when the latest Apple I auction ends, its price could reach even new heights.

“That unit didn’t have the provenance that this board has,” says Cohen of the Apple I that The Henry Ford Museum bought. “This Apple I is a unicorn.”

According to Cohen, the latest Apple I appears to have been one of only a few prototypes Apple built in its early days and comes with several different components added by Apple that were unlike later versions currently in the Apple I registry—a registry of all the known Apple I computers in the world.

“This particular board looks like it was used as an experiment on memory and how much cheaper you could buy memory,” Cohen describes. “It was also used in experiments on getting ‘cleaner power’ to the memory chips.

In other words, it’s exceedingly rare, and for historians and Apple I-seekers, it’s exceedingly attractive.

“There were some odd experiments on the board that someone like Steve Jobs or one of the Apple folks would have done,” Cohen adds. “It’s not something that would’ve been done by a hardware hacker back in the day.”

What’s interesting, however, is that only longtime Apple I historians who know for what they’re looking understood what they were looking at when the computer was first pitched for an auction.

The current, anonymous owner—who is donating a portion of the proceeds to The Leukemia & Lymphoma Society after it’s sold on auction site CharityBuzz—bought the Apple I in January 2000 for just $18,000, the going rate at a time when Apple’s history wasn’t so important to historians. It seems to have been used by the previous owner to create a Spanish language program to teach people how to speak Spanish, Cohen says.

But there’s more to identifying a “unicorn” than simply knowing that some components on a board weren’t like the others. The Apple I also comes with a cassette that runs the software required to actually use the computer. On it, Cohen found some writing, which he says, offered some clues on the computer’s value.

According to Cohen, the Apple I up for auction on CharityBuzz was assembled in part by Kottke during that summer of 1976.

“The cassette tape included in the collection has Dan’s handwriting, which allowed us to date the cassette,” Cohen says. “Pretty much summer break 1976.”

He was able to track down Kottke, who confirmed to CharityBuzz that the writing on the cassette was indeed his. Fortune was unable to reach Kottke for comment.

But Kottke’s story doesn’t quite end there. In fact, he might have one of the more interesting stories in Apple lore.

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In addition to building what might be the most valuable Apple I computer in the world, Kottke went to India with Jobs years prior while they both sought enlightenment. Despite being one of Apple’s first employees (he was officially Apple employee #12) and working on countless Apple machines over the years, Kottke is also remembered for being one of the early employees at Apple who Jobs denied stock.

Woz, however, thought better of the idea.

“He’s the one who didn’t get Apple stock from Jobs because he didn’t want to give it to anyone who wasn’t officially an engineer, but Woz gave him stock,” Cohen says of Kottke.

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Still, his decision to work at Apple over summer break from college has proven fateful. It’s impossible that a young Kottke could’ve known that one day, he’d be working on an Apple I board worth at least $1 million.

The Apple I auction ends on Thursday. As of this writing, the current bid is $510,000.