The explorer's watch is not a military field watch with a different name: the field watch tells time in battle, the exploration watch must do more, in conditions that no military regulation ever anticipated. The distinction is real, not marketing. Rolex's Explorer set the genre canon in 1953, but the model that generated the richest technical vocabulary is the Explorer II, in its first version ref. 1655.
The ref. 1655 is what everyone calls the «Arrow Hand», from the profile of the orange 24-hour hand, wide and pointed, readable even with gloves. It's a precise nickname: that hand is the identifying element of the model. That said, the 1655 is not a GMT in the technical sense of the term. The 24-hour hand is not independent of the hour hand: the two move synchronized. The orange hand sweeps across the fixed 24-hour bezel — steel, non-rotating, Arabic numerals for even numbers and hash indices for odd numbers — and only indicates whether the local time is daytime or nighttime. A day/night indicator, not a system for reading two separate time zones. This actual function will arrive with the ref. 16550, in 1985.
On the 1655 the outer bezel could not receive luminescent material: tritium on exposed surfaces deteriorated too quickly. The solution was to add 24 tritium indices directly to the dial, corresponding to the numbers on the 24-hour scale. You just had to remember that 12, 3, 6 and 9 were six hours apart on the scale. An intelligent compromise solution that tells much about the metallurgy and chemistry of the period.
The polished white dial of the Explorer II — the so-called «polar dial» — is a chromatic variant, not a separate line. The term «Rail Dial», on the other hand, is specific to the ref. 16550 with cream dial: it designates those examples in which the «C» of «Chronometer» aligns vertically with the «C» of «Certified» in the dial text. A minimal graphic particularity, sought by collectors for reasons that have more to do with statistical rarity than with readability. In the ref. 14270, the «Blackout» variant designates the first series with numerals 3, 6, 9 filled with black enamel: the most expensive among the variants of that reference.
With the ref. 216570 (2011) Rolex introduces the Maxi markers, enlarged to compensate for the dial grown to 42 mm. On the polar the outline remains black; on the black version the markers are in white gold. The Chromalight, luminescent with blue emission, enters the Explorer line paired with the 3230 caliber. The marking «T SWISS T» on older dials indicates luminescent tritium; «T SWISS 25 T», introduced on the Mark IV, specifies an activity below 25 mCi according to the safety regulations of the time.