Some interesting information for those that didn't know:
What Do The Numbers Mean... Model numbers explained
Rolex model numbers currently four, five or six digits long. The first two or three describes the type of watch. If the watch has five digits, the second from the last describes (some exceptions) the type of bezel and the last describes the material that the watch is made from.
Typically the reference number only changes when major changes are introduced to the watch. If only minor changes are made a "1" is added to the beginning of the old reference. So in the early 2000s when the 14270 Explorer was remodeled only slightly it became the 114270. Major changes such as the introduction of a larger case or major cosmetic differences will result in a totally new reference number all together.
Type of Watch:
Oyster Perpetual - 10, 140, 142
Airking - 55 & 140
Date - 15 & 150
Datejust - 16 & 162
Daytona Manual Wind - 62
Daytona Cosmograph - 165, 1165
Explorer II - 165
Sea Dweller - 16 & 166
Submariner - 16, 166 & 168
Submariner (no date) - 55 & 140
GMT Master - 16, 65, 167
GMT Master II - 167, 1167
Oysterquartz Datejust - 170
Oysterquartz Day-Date - 190
Day-Date (President) - 65, 66, 18, 180, 182 & 183
Yachtmaster - 166, 686 & 696
Midsize Oyster Perp DJ - 68, 682
Ladies Oyster Perpetual - 67, 671, 672
Ladies Date - 65, 69, 691 & 692
Ladies Datejust - 65, 69, 691 & 692
Type of Bezel (does not include Sports/Professional models)
0 = Smooth/Domed
1 = Engine Turned
2 = Engraved
3 = Fluted
4 = Bark
5 = Pyramid (Not Used)
6 = Turn-O-Graph
7 = Various
8 = Special (Not Currently Used)
9 = Open (Not Currently Used)
Type of Material...
0 = Stainless Steel
1 = Yellow gold Placor (Tudor)
22 = Stainless steel and Platinum (Yachtmaster)
3 = Stainless Steel and 18k Yellow gold (Yellow Rolesor)
4 = Stainless Steel and 18k White gold (White Rolesor)
5 = Gold Shell (No Longer Used)
6 = Platinum
7 = 14k Yellow gold (No Longer Used)
8 = 18k Yellow gold
9 = 18k White gold