Stephen King’s assistant has solved the mystery of a manuscript that an Auburn bookstore manager had hoped was a working version that eventually became “Under the Dome,” a King novel that was turned into a TV series.

Marsha DeFilippo, the Bangor horror writer’s longtime assistant, said Sunday that the manuscript sold to Artios Books last month was definitely a copy of an earlier version of “Under the Dome” titled “The Cannibals.”

The 120 pages bought by bookstore manager Walter Lantz were downloaded from King’s website, DeFilippo said. The original is in King’s office, she said.

“I think the mystery is solved,” she said. “We scanned in original pages with Stephen’s handwritten edits. There are two PDF documents (on his website) that match exactly the 120 pages (Lantz) had.”

Lantz said the pages were part of a King collection sold to the store by an Auburn woman trying to raise money to pay for a relative’s funeral.

DeFilippo noted that King, who has penned more than 50 novels and hundreds of short stories, has lost manuscripts from time to time.

“Some of them got lost over the years between their moves, and I am sure in the early days he may not have kept everything he ever had,” DeFilippo said.

Nevertheless, she said it would have been very unusual for a King manuscript to land in a bookshop in Maine.

“I would say it is uncommon, which is why it triggered my curiosity,” said DeFilippo, who talked with Lantz by phone on Friday.

DeFilippo said King has donated many of his original papers to the Fogler Library at the University of Maine in Orono.

“Most of it is already at the University of Maine,” she said.