225 Slant Harmonic balancer

If you're sure it's an '81 model, they made 2 different ones that year. If it was originally in an automatic trans vehicle, it needs to be one from a '78 and newer with an automatic trans. If it came from a manual trans vehicle, it needs the '79 and newer manual trans version. They also made a few oddball versions with built-in 2 and 3 groove pulleys in the outer ring, but, yours looks to be the type with bolt-on pulleys.