The birds illustrated  are known as the frizzle or curled feather variety. The flight feathers are soft and trail to the ground. This would be a lethal mutation in the wild because the birds cannot fly. 

Birds which are probably heterozygous for this mutation have curled feathers trailing only from the scapular (shoulder) and thigh coverts. The rest of the body feathers appear normal. The flight feathers are slightly fluted, and the tail feathers are also affected. This type (not illustrated) is known as the smooth-breasted. 

The best breeding combination is a mixture of the two types; such a pair will breed both types. 
The curled-feather and the smooth-breasted are both standardized in the UK. 
In Germany it is just the smooth-breasted that is standardized.  In the USA, it is the curled-feather variety. 

Sebastopols are a light weight domestic goose weighing 10-16 lbs. Temperament varies depending on the strain, as does egg-laying capability. They were introduced into Britain in 1859. Birds like these were reported from the countries surrounding the Black Sea, and the lower Danube - hence an alternative name: the Danubian goose.