The Rolex reference 5100 in yellow gold is one of the rarest Rolex watches ever produced, estimated at only 900 pieces made. Moreover it is a proper milestone in the history of Rolex and swiss watchmaking in general as it was the first watch to feature the famous Beta 21 movement. This movement was created at the end of the 1960s, when 21 of the most important maisons in horology teamed up their best engineers and technicians to create the first and most precise swiss quartz movement in order to compete with the japanese competition of the time. 

The watch was made of solid 18K gold and had a 40mm case which was extraordinarily large for its time. There are several more things that are quite notable of this watch: It was the first Rolex watch to ever feature a sapphire crystal as well as a quick-set date complication. It was in production for only 2 years and all models were sold in pre-order before the production even began.