[schoolboy]

Originally Posted by AEC

006 features a sapphire crystal over the dial (in place of the acrylic crystal used on 005), and it has a transparent case-back (in place of the steel case-back and soft iron dust cover used on 005). The movement used in 006 has slightly upgraded finishing, since it is meant to be admired through that transparent case-back.

There is plenty of discussion to be found on the internet about who prefers which of these two versions of the Speedmaster Professional and why.