I often see people incorrectly refer to the Canon metal lens mount as "Stainless Steel" and want to correct this misconception. (the camera mount does appear to be one solid material, so it would be easy to assume it is Stainless Steel, as is often reported)
I have taken apart a couple of lenses and discovered they are not Stainless Steel. If you remove the metal lens mount from a lens, and turn it over, you can easily see that they are made of Brass with some kind of plating applied.
The area of the mount that mates to the lens itself is left un-plated, or machined to an exact thickness. Various thickness shims can also be added for critical focus adjustment.