Rolex crown seals have evolved through three generations: the single-dot 'Twinlock' (Datejust and dress models), the two-dot 'Twinlock' variant, and the three-dot 'Triplock' introduced on the 5513 and 1665 for professional dive watches. Correct crown for reference and serial range is a critical originality checkpoint. The earliest 'Big Crown' 6538 Submariners used an oversized 8mm crown without shoulder guards—original Big Crowns are worn to nubs on most surviving examples and correct replacements are among the rarest parts in the entire Rolex universe.