Officine Panerai, founded in 1860 by Giovanni Panerai, was the first distributor of fine Swiss watches in the Florence region of Italy. The company didn't gain historical importance until Giovanni's grandson, Guido Panerai, become involved. In the early 1900's Guido began experimenting with luminous materials and developed a method for making optical equipment luminous by the use of a material he invented called Radiomir. As a specialist supplier of precision optical equipment Officine Panerai were granted the license to supply the Italian Royal Navy with various specialized instruments such as compasses and timing devises. Officine Panerai were therefore the obvious choice when the Navy wished to provide its elite divers with wristwatches. Panerai then turned to Rolex, who manufactured the most reliable waterproof wristwatch of the era, to assist them. In 1938 Panerai launched the Ref.3646, using a Rolex movement and Oyster Watch Co. case (a division of the Rolex Watch Company) and fitted with a sandwich construction dial made by themselves with a base plate of Radiomir and an upper black plate with cut-outs for numerals. The 3646 proved a success when tested in the field and issued to the Italian Navy divers. After 10 years of use and more widespread disbursement, further developments were considered important and the model updated with more robust shoulders, although initially released in a new case with the standard Oyster locking crown used by Rolex, later versions were fitted with a lever design locking crown patented by Officine Panerai that enabled the watch to descend to 200m and be wound under water if required. The new revised model was given the reference number 6152. Examples of the Ref.6152 issued to the commandos, frogmen and specialized SLS torpedo riders of the Italian Navy were allowed to have the prestigious title of "Marina Militare" printed on the dial.