Jeff Wells picked up the copy of Edgar Allan Poe’s The Tell-Tale Heart, a 20-minute film made in 1953, while rifling through a “bargain box”.
He bought the British adaptation of the film in 1984 from a shop in Brighton, Sussex, where he was working as a nurse.
But when Mr Wells, 65, moved to Drummore, near Stranraer, Dumfries and Galloway, in 2002, he stored the film in the attic without realising its significance.
Meanwhile Adelphi Films, which owns the rights to the 1953 horror, was trying to track it down to include it their archive at the British Film Institute (BFI).
Mr Wells only realised their manager Kate Lees had been searching for the film for 12 years when he tried to sell it on eBay for £5.
He said: “I bought the film in Brighton in 1984, along with some others in a bargain box for £5. At the time it was a hobby of mine to have these 16mm films.
“I didn’t know the significance of it when I bought it, I just picked it up randomly.”
When he moved, Mr Wells took the film with him as it was “particularly interesting” but forgot about it until October 2017.
Researching the one reel 16mm print he realised it was missing from Adelphi Films catalogue and contacted them to confirm the significant find.
Now, a year later, the film will be available for the public to view as the BFI has put it online.
Mr Wells said: “Everything from the move just went in the loft and it wasn’t until last year when I was clearing out some things I came across it again.
“I was going to try and sell it on eBay for £5 when I saw Adelphi Films was looking for it.
“And lucky I did because it would have been sad to lose or sell to someone else who didn’t know the significance of it.
“I was stunned when I saw it was the same one I had, I really couldn’t believe it, I thought it can’t possibly be the one I have they are looking for.
“Its discovery was a happy accident.” In November, Ms Lees drove 400 miles from London to Mr Wells’ home to collect the film to add to a collection preserved by the BFI.
The 16mm print and a digital copy of the restoration has been donated to the BFI, and the film will be accessible via the BFI Player from 26 October for two weeks.