Ridden by Adam Wedge, for whom it was a first Festival win, seven-year-old Lisnagar Oscar beat 20/1 shot Ronald Pump by two lengths, with 33/1 Bacardys in third.
Curtis said: "He ran a blinder on his last run and I am not sure whatever happened to him at the beginning of the season. We did find a few ulcers and things, so we treated him for that and he was so well at home. I thought 50/1 was a massive price.
"Adam is what I call such a solid, solid jockey. I am delighted for him to get his first Festival winner. He has given the horse a brilliant ride.
"I'm absolutely delighted. I thought he had a big chance today; I didn't think he would beat [2019 winner] Paisley Park, but I thought he'd get placed. To see him improve that much - I'm just delighted. I was thinking, 'don't get caught, don't get caught.' He took a nice pull all the way and I thought they went a good gallop.
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"It's a time when I need it; I'm trying to rebuild the yard. I've had four winners here before, but it's been five years, so it's just great to win another one."