Four decades later and Mr Leonard's grieving wife still believes this man had something to do with the shooting of her husband.
"I was a wife, a mother and a widow all in one year. I was convinced, and still am, that this particular person had something to do with the whole thing," said an emotional Betty Leonard, who has never got over the death of her "soul-mate".
"I think often about how different my life would have been had Louis lived because we had massive plans, and one of them was to have lots of children. I did intend to have a large family and that was denied to me. His plan was to open up several butcher shops, he was a born businessman and he had the tenacity and the motivation, the love of hard work, and all of that combined, and I think with me behind him he would have succeeded," she told The Impartial Reporter.
"He wasn't just my husband, he was my soul-mate and I suppose I didn't understand that term until in recent years. He used to phone me every day; we were just a pair."
On the night of December 15, Mrs Leonard saw her husband alive for the final time. After a shopping trip with her sister, she returned to the butcher's shop to help prepare for deliveries.
"It was 9pm when I got there. I remember two young fellas came in to chat Louis about football. I was trying to get parcels ready and one of them said: 'Are you not afraid about being robbed Louis?' And he lifted the knife and said: 'I have the tools and I know how to use them'. Those were his last words."
She recalls, with much anxiety, the moment a local man walked into the butcher's shop and threatened to shoot her husband days before. Believing the man had something to do with his death she sought a meeting with him a month after the murder.
"I asked the investigating officer to let me meet him; he brought him to my house and I asked him about it. And do you know he said to me? 'Worst things could happen... keep your chin up'. He then shook the hands of all my family and left. I can't remember anger being an emotion then, but I was grieving, it was beyond grief."
Following the murder, Mrs Leonard found it difficult to cope, having to bring up the couple's only son Tony, run the business and look after her ill mother.
"When I used to go to bed at night everything was nice and bright in my dreams. When I woke up everything was black -- reality had hit. I used to be driving along; I wasn't able to pray and I remember screaming to God; 'How can I keep going?'. I had to grow up very quickly. I was only 26 and I remember the clergyman asking me how I was and I said: 'I feel like an old woman'. I really did feel like 108 at that time. It was very difficult."
Mrs Leonard, who gets a lot of joy out of spending time with her grandchildren (pictured), says it is her and her family's "main aim" to keep calling for justice for Louis.
At the weekend, a number of events took place in Fermanagh. On Friday, a play entitled 'Troubled Conversations' was held in the Knocks Community hall. On Saturday, family and friends took part in a memorial walk in Derrylin, addressed by veteran civil rights campaigner and former council chairman, Paul Corrigan. Then an anniversary mass was held at St. Patrick's Church in Donagh with a speech afterwards by Peter Quinn, family friend and former president of the GAA. On Sunday, a short wreath laying ceremony was held at Louis' grave followed by the unveiling of a memorial plaque at the GAA grounds.