The outer surfaces of the handle and body are identical to the shoulder. They are protected by a nickel coating against corrosion. The inner surface of the body is not coated, however, and at high magnification the grain structure and micro-porosity can then be seen.
This indicates that the surface has not been machined (i.e. formed by a cutting or grinding process), and instead was cast (by injecting molten metal into a shaped mould). A metal mould, which has a smooth surface, is most likely for mass-produced items such as this corkscrew. This technique is called die casting. Sand casting is used for larger components, in which the shape for the metal component is formed from sand, but the sandy surface of the mould would leave a much rougher surface. Casting as a production method is also indicated by the microstructure evidence and the porosity.
The cast component has probably been pickled by acid to clean it before the corrosion resistant coating was applied electrochemically. This improves adhesion of the coating and the acid also dissolves some material to reveal the microstructure. The inner surface has not been protected by Nickel as it is less exposed to corrosive salts (from skin contact), and the appearance of the inner surface is not important. Reducing the amount of nickel used reduces the cost of the component.