They knocked on the door of a farmhouse belonging to the Brierley family and were allowed to pitch their tent in the field at the rear of their bungalow.
After a downpour on their first night, the pair sought refuge in the farmhouse and ended up staying the week.
McCartney and Harrison became friendly with the young John Brierley, a musician himself, and later sat in with his local skiffle group The Vikings during a performance at the Queen’s Hotel pub in the village.
Mr Brierley also recalled several days of playing snooker and playing Elvis Presley’s first LP, Rock ‘N’ Roll, as well as McCartney banging away on the family piano trying to work out the solo in the middle of the Buddy Holly song Think It Over.
The pair enjoyed themselves so much they returned to the Brierleys’ home the following year and later made off with one of the family’s blankets.
That stuck in the memory of Mr Brierley’s mother, Irene, long after Beatlemania swept the globe and years later in the 1980s she wrote to McCartney to jokingly remind him she was still a blanket short.
McCartney responded graciously as he wrote back: “Dear Mrs Brierley (Irene), Your letter reached me eventually and I was sorry to hear about my ‘debt’.
“I remember well the fun we had in Harlech and hope the enclosed cheque will settle our money differences!
“Excuse the scrawl as I’m trying to write this on a bouncing express train!!
“I was very sorry to hear about your hubby – he was a very nice man. Please give my best to your boys.
“Kindest regards, Paul (McCartney).”
Mrs Brierley went on to cash the £30 cheque sent by the musician.
The letter has a starting price of £3,000 and is being auctioned at www.tracks.co.uk, which offers free valuations of all types of Beatles and music memorabilia.