151 US proof is 76 percent alc/vol, known in Canada as 30 degrees over proof. It's a fairly common strength, e.g. Wray and Nephew in Jamaica.

When I lived in British Columbia back in the 1970s. they sold 3 strengths of Guyanese (Demarrera) rum: 30 under proof (40%). proof (54%) and 30 over proof (76%).

One thing I noticed doesn't make any logical sense. If you mix 1 oz of over-proof rum with 7 ounces of Coca-cola, it will hit you more than if you mix 2 ounces of under-proof with 6 oz of Coke, despite the fact that both drinks are the same size and the over-proof drink has less alcohol.

I guess that's why they call it spirits.