The purpose of this topic is to attempt a clear cataloging of the casebacks that were used on the Sea-Dweller 16660. The subject of the 16660 casebacks is not very well known and, to my knowledge, has never been addressed with such detail. To create an accurate classification and correct chronological placement of the casebacks for the 16660 reference, it was necessary to conduct a dual analysis: one on the external engravings of the caseback and another on those inside. The reason for this approach lies in the fact that, almost never with the 16660 reference, does a specific external engraving correspond with a specific internal engraving. However, with this dual analysis method, it was possible to identify and discover the correct external-internal combinations for all serials of the 16660 reference.
Let’s first examine the types of external caseback engravings that appeared over the years.
FIRST TYPE EXTERNAL CASEBACK (estMK1)
Characteristics:
- Only a single crown is present, located between “Rolex” and “Oyster”.
- The word “Original” is absent before “Gas Escape Valve”.
- The “G” of “Gas” is much more open and elongated compared to later series.


SECOND TYPE EXTERNAL CASEBACK (estMK2)
Characteristics:
- Two crowns are present, located between “Rolex” and “Original” and “Oyster” and “Valve”.
- The word “Original” appears before “Gas Escape Valve”.
- The “G” of “Gas” is open and elongated, but less so than in the first series.


THIRD TYPE EXTERNAL CASEBACK (estMK3)
Characteristics:
- Two crowns are present, located between “Rolex” and “Original” and “Oyster” and “Valve”.
- The word “Original” appears before “Gas Escape Valve”.
- The “G” of “Gas” is much more rounded.
- The serifs almost completely disappear, giving the engraving a more modern look.
- The engraving tends to be closer to the knurling of the caseback.
- Some examples have a much lighter engraving compared to earlier series.


FOURTH TYPE EXTERNAL CASEBACK (estMK4)
Characteristics:
- No external engraving is present.


At this point, we have cataloged four types of external engravings, summarized below:


Having understood and assimilated the graphic differences that characterized the external part of the casebacks in the various production batches of the 16660 reference, it’s now time to analyze the internal engravings. The internal casebacks can be divided into three different series,
- Caseback with “Patented” engraving
- Caseback with “Patented” engraving milled
- Caseback without “Patented” engraving
For those unaware, the reason for these modifications is that at the beginning of the 1980s, Rolex’s patent for the famous Oyster case expired. For legal reasons, after the expiration of this patent, Rolex was no longer authorized to use the term “PATENTED” on its casebacks.
Therefore:
- Until the early 1980s, not only on the 16660 but on all Rolex Oyster cases, the word “PATENTED” was ALWAYS present on the casebacks.
- For a brief transitional period, on casebacks produced and engraved before the patent expired, the word “PATENTED” was milled off.
- After these “transitional” casebacks were used up, the word “PATENTED” was no longer engraved on casebacks and disappeared entirely.
Now, let’s examine the three series of internal casebacks in detail.
FIRST SERIES INTERNAL CASEBACK – PATENTED (intMK1)
Characteristics:
- The word “PATENTED” is present.


SECOND SERIES INTERNAL CASEBACK – MILLED PATENTED (intMK2)
Characteristics:
- The word “PATENTED” is milled off.


THIRD SERIES INTERNAL CASEBACK – PATENTED ABSENT (intMK3)
Characteristics:
- The word “PATENTED” is not present.


We have thus cataloged three types of internal engravings, summarized below:


At this point, we have classified all the types of external and internal engravings on the caseback that alternated over the years.
It is now time to combine this information and create an accurate chronological classification that will allow you to determine if the caseback of your 16660 is correct for the case serial number.
Let’s begin.
If the watch’s serial number is below 6.6 million, the only correct external-internal combination is this:

If the watch’s serial number is between 7.3 and 7.5 million, the correct external-internal combinations are these:

If the watch’s serial number is between 8.0 and 8.5 million, the correct external-internal combinations are these:

If the watch’s serial number is between 8.5 million and R5, the correct external-internal combinations are these:

SERVICE CASEBACKS
For the replacement of casebacks prior to the end of production of the 16660, it is believed that casebacks chronologically newer than the serial number of the series were used.
The caseback we will now examine was produced concurrently with the end of the production of the 16660 reference and is officially considered as a service caseback.
- Two crowns are present, located between “Rolex” and “Original” and “Oyster” and “Valve”.
- The word “Original” appears before “Gas Escape Valve”.
- The word “PATENTED” is absent from the inside of the caseback.
- The “G” of “Gas” is more closed and rounded.
- The engraving “ROLEX SA” is present.
- The internal caseback engraving differs completely from previous series.

I also add that if anyone reading this is aware of, or possesses photographic material regarding, other types of casebacks that I have not listed, it would be very interesting to see and analyze them.
In particular, I was unable to find material on the First Series caseback with milled “PATENTED”. In my research, I did not encounter any, but I believe it is possible that such a caseback may have existed.
This article was written by me and published on the website of Roberto “Jatucka” Randazzo in 2019. I thank Roberto for allowing me to add it to the blog of my website.