The first Rolex I bought to wear is the Submariner. Recently I sold it to to an Indonesian collector to fund a mint discontinued Sea Dweller.
The earlier Sea Dwellers, I feel, are becoming increasingly harder to come by, especially in mint condition with full box and papers and a recent completed full service, like this example.
One day, I will buy back the Submariner as she is still readily available in the market but the price :)
The Original Gas Escape Valve, a Rolex patent, is shown on the side of the case. It is a circular piston made of titanium, which appears slightly darker than the stainless steel case.
The Sea Dweller is a professional Divers watch that is used by COMEX divers in an extremely hazardous and unforgiving environment deep below sea level. It is noteworthy that the steel grade used in Sea Dwellers is 904L vis-à-vis the 316L used in the Submariner (I stand corrected), increasing the Sea Dweller's corrosion resistance.
As you know, I am no mechanical person but here are two beautiful pictures of the Caliber 3135 (taken from the Internet):
This shot is from "The Rolex Submariner Cal. 3135" by Andrew Babanin.
I will never be able to do this to my Rolex!
This shot is from "Rolex Caliber 3135 – Still worthy of the crown after all these years?" by A.Watchmaker.