In 1971, Rolex introduced the Explorer II as a larger sibling to the original Explorer I wristwatch. The first reference was the Explorer II ref. 1655, featuring a 39mm stainless steel case and matching steel bracelet. These watches were designed for spelunkers, with key features such as an extra 24-hour hand that points to a fixed bezel with 24-hour markings. The watch was not a dual-time timepiece, but later changed to become a dual-time timepiece.
Transitional references, such as the Explorer II ref. 16550, introduced features of preceding versions along with additional modifications that pave the way for future references. The 1985 model introduced the larger 40mm case size, sapphire crystal, and Caliber 3085, revamping the dial to look similar to other sports models with Mercedes-style hands and the combo of triangular, round, and rectangular lume-filled hour markers.
In 1989, Rolex unveiled the ref. 16570, which had many of the same features as the preceding ref. 16550 with notable upgrades. The ref. 16570 was made for 22 years and underwent several changes over its production run. Early models ran on Caliber 3185, later models housed Caliber 3186, and the luminescent material changed several times. Rolex began engraving the rehaut (space between the dial and the crystal) with "ROLEX" starting in the mid-2000s as an anti-counterfeiting measure. Later examples of the Explorer II ref. 16570 are notably different from early examples of the same model.
