Bob Holmes, 76, was gobsmacked when he discovered a betting slip backing Red Rum during a clearout of his deceased father-in-law's belongings.
The uncashed slip from 1974 wagered £1 on one of the greatest racehorses in history at odds of 11-1.
That year would be Red Rum's second in three Grand National victories but Mr Holmes' father-in-law Joe Robertson would never collect his £12 winnings.
Mr Holmes and his wife Nancy, 75, were helping his sister-in-law Rhoda Robertson, 79, move house in Houston, Renfrewshire, last November.
Their curious discovery caught the imagination of betting giant William Hill who honoured the 43-year-old bet and even adjusted the takings for inflation.
It meant the family collected £130 from the bookmakers as well as an equivalent amount in free bets for this weekend's race.
Bob Holmes has taken winnings from a Grand National bet placed 43 years ago Grand National runners 2017 Thu, April 6, 2017
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The company also donated £130 to Water Aid – Bob's charity of choice.
I found a small slip with "William Hill" written at the top of it which piqued my curiosity
The bet had been traced back to a William Hill branch in Paisley which now no longer exists.
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But the pensioner says that he will not be leaving behind a slip for his three children if the horse he has backed this weekend, Blaklion at £20 each way, is the first to cross the finish line.
He said: "We were raking through my father-in-law's papers on the off-chance that there might have been something worth holding on to.
The uncashed slip from 1974 wagered £1 on Red Rum at odds of 11-1
"In amongst the various old income tax papers, I found a small slip with "William Hill" written at the top of it which piqued my curiosity.
"On closer inspection I noticed that it was backing Red Rum which I found even more curious.
"I thought about framing it or giving it away to a collector but felt it was worth asking William Hill if they would honour the bet.
"They said they were legally obligated to but went above and beyond what I expected of them.
"If my free bet comes in then I'll be taking my winnings immediately. There's been no paper exchanged for this so I have nothing for my children to find in 40 years."
Mr Robertson, who died at the age of 85 in 1978, was similar to Bob in that he only placed a bet about twice a year.
Mr Holmes' father-in-law Joe Robertson never collected his £12 winnings
He would bet on the two major horse races – The Derby and Grand National – while Mr Holmes normally only wagers on major golf tournaments a few times a year.
Mr Holmes, a former foreign office worker, emailed William Hill with a scanned copy of the betting slip which was soon authenticated as it had been stamped at the time.
With their free bets, in an homage to Red Rum, Nancy has backed Definitely Red at £25 each way while her sister Rhoda has backed Ucello Conti at £20 each way.
Mr Holmes speculated that his father-in-law must have searched all over the house for his winning paper.
He added: "I think he must have lost it – then turned the house upside down looking for it.
"It's good fun. We've divided the winnings evenly among the three of us and hopefully we'll have some more money."
William Hill honoured the 43-year-old bet and adjusted the takings for inflation
William Hill spokesman Graham Sharpe said this smashed their previous record of uncollected winnings.
He said: "It's difficult to understand how someone who backed Red Rum would not have collected their bet as anyone who did usually boasts quite a bit about backing the Grand National winner.
"Our previous record for an uncollected bet was in the 1970s when a woman only felt able to go through her late husband's clothes after seven years and found a slip in his suit pocket.
"But Bob's win smashes that by quite a distance.
"What I remember about Red Rum was that he would regularly take us to the cleaners in the 1970s and here he is more than 40 years on still doing it. "