This is the front pump seal on the input shaft of the gearbox.

This seal is very prone to leaking causing the characteristic oil leak from the underside of the bell housing.

When changing the gearbox front pump seal GM8670283 the below parts are also usually changed:

GM8655280 x6 required
GM24210605 x1
GM8623978 x1

All models with GM400 gearboxes from 1966-1980 All Models

1980-1992 Continental, Corniche, Eight, Mulsanne, Spirit, Spur, Turbo (up to approx. VIN 40000)

Supersession: ACD8670283, GM8626916