After the successful ascent to the top of Mount Everest on the wrist of Sir Edmund Hillary, Rolex’s Explorer enjoyed a growing popularity, so much so that they began to produce variations. The reference 5500 was launched at the end of the 1950s, and it was essentially an Air King with Explorer dial.

The present watch is an extremely collectible example because of its superlatively well-preserved dial, and unusual signature combination. It presents both the underline mark and the T<25 designation. In the beginning of the 1960s, regulations imposed watchmakers to discontinue the use of radioactive radium, and substitute it with the safer tritium. With this change, watchmaking brands were unsure on how to communicate this to the public, Rolex, for example, introduced the underline mark to indicate radium-free dials, then switched to T<25 Swiss, and eventually settled on T Swiss T. The present piece perfectly exemplifies this moment in time, as it presents both the underline mark and the T<25 designation. Furthermore, close analysis shows both markings were printed at the same time in silver on a gilt “single swiss” dial: obviously, Rolex employed a pre-existing dial and modified with both designations.