Prospex for Seiko basically means "Professional Specification" that their timepieces in this category  are built with very high standards to be used by professionals. One of the most popular releases in recent years is probably the remake of the Seiko 6309 (circa 1976-1988) commonly known as the turtle. The original 6309 divers are pretty rare given its short 12 years production lifespan. Although there are many exmples in the market, there are less than a dozen that are original.