The Hidden World of the Casio DW-5600C Screwback Cases
When collectors talk about vintage Casio G-Shocks, the conversation often begins with the obvious details: the dial, bezel condition, module, or overall appearance.
But underneath the watch is another fascinating world.
The screwback case of the DW-5600C contains a surprising amount of history. The module, caseback finish, Japan marking, engraving style, and small construction details reveal different production periods and manufacturing variations.
For collectors, the back of the watch is not just a cover.
It is a fingerprint.
Two Modules, Two Eras
The Casio DW-5600C was produced from the late 1980s into the early 1990s and is mainly found with two different modules: the earlier Module 691 and the later Module 901.
The Module 691 was introduced first, while Casio later replaced it with the Module 901 around 1990.
From the outside, the two modules look almost identical and provide nearly the same functions. The biggest changes are found inside the watch.
The 691 module uses a CR2320 battery, while the 901 uses the smaller CR2016 battery. This required a different battery spacer and a revised rubber cushion between the module and the screwback.
However, the stainless steel screwback case itself remained the same. Both modules fit the same case construction.
This is where the story becomes interesting.
Mirrorback and Circleback
One of the easiest details collectors use when identifying a DW-5600C is the finish of the screwback.
Most early Module 691 examples are associated with the highly polished mirrorback finish.
Later Module 901 watches are normally found with the circleback finish, identified by the fine circular machining marks on the stainless steel back.
Because of this, many collectors create simple rules:
- 691 = mirrorback
- 901 = circleback
But vintage Casio is rarely that simple.
When the Rules Do Not Apply
Some DW-5600C watches do not follow the expected patterns, and these exceptions are often the most interesting discoveries.
The 901 Module With a Mirrorback
One of the most surprising combinations is a Module 901 fitted with a mirrorback caseback.
Since the mirrorback is usually connected with earlier 691 watches, these examples can easily be mistaken for a different model.
Only opening the watch and checking the module reveals the truth.
These transitional examples show that Casio did not always change every component at the same time. Remaining parts, production batches, and factory transitions created combinations that do not fit the usual collector charts.
The 691 Module With a Circleback
The opposite combination is even more unusual.
Although the 691 module is normally associated with the polished mirrorback, some examples are known with a circleback case.
This proves an important point:
The caseback finish can provide clues, but the module number remains the final confirmation.
Japan Case Variations – H, A, S and V
Another important detail is the letter stamped after JAPAN on the screwback.
These letters are not random markings. They represent different case variations and manufacturing differences.
Japan H – The Standard Production Case
The Japan H case is the most common variation found on vintage DW-5600C models.
It was produced with both the 691 and 901 modules and can be found with different caseback finishes.
The H case represents the standard production version that allowed Casio to manufacture large numbers of DW-5600C watches during the golden era of screwback G-Shocks.
Because so many were produced, it remains the easiest version to find today.
Japan A – A More Refined Case
The Japan A case is less common and shows a higher level of finishing compared to the H case.
The machining is cleaner, and subtle differences in the case shape and finishing give it a more refined appearance.
Many Japan A examples include a serial number, which adds another interesting detail for collectors.
Finding an early 691 Japan A is especially attractive among enthusiasts.
Japan S – The Premium Case
Among vintage DW-5600C collectors, the Japan S case is often considered one of the most desirable variations.
The machining quality is noticeably higher, with cleaner surfaces and a more refined finish.
Both 691 and 901 module Japan S examples exist, but they are difficult to find.
One identifying feature is the placement of the Shock Resistant engraving on the caseback, which is positioned differently compared with many other versions.
The S case represents a period when Casio paid extraordinary attention to manufacturing details, even on a rugged tool watch.
Japan V – The Mystery Case
The Japan V case remains one of the most mysterious DW-5600C variations.
Very few examples have been documented, making it one of the hardest versions for collectors to discover.
The shape is closer to the H case, but it has its own unique characteristics.
Finding a Japan V is not only about rarity. It is about preserving a small and unusual piece of G-Shock history.
Japan S Cases With Different Modules
The Japan S case also demonstrates why vintage collecting cannot always be reduced to simple rules.
Both 691 and 901 module Japan S examples exist.
Some collectors assume that premium cases belong only to a specific production period, but Casio continued using these higher-quality cases during the transition between modules.
Finding an S case is already difficult.
Finding the correct combination of case, module, and caseback finish makes the search even more interesting.
Three Different Caseback Layouts
The DW-5600C screwback was also produced with different engraving layouts.
The earliest style includes:
- "200M Water Resistant"
- "ST. STEEL"
Later versions changed the engraving to:
- "Stainless Steel"
- No 200M marking
The rarest layout even removes the word JAPAN entirely.
Some collectors believe these examples may represent unusual assembly periods, as several known watches with this style contain battery retainers marked KOREA.
These small details add another layer to the history of the DW-5600C.
Why Did These Variations Exist?
Casio produced G-Shocks on a large scale, and production changes did not always happen instantly.
Factories used remaining stock, different production batches, and updated components during transition periods.
A new module did not always mean a completely new case.
A new caseback finish did not always appear at the same time as other changes.
This is why vintage G-Shock collecting is full of exceptions.
The patterns help collectors understand these watches, but they should never replace careful examination.
The Lesson for Collectors
A vintage DW-5600C should never be judged by only one detail.
The module, caseback finish, Japan marking, serial number, engraving style, and construction details must all be considered together.
Two watches may look identical from the front, with the same bezel, strap, and display, but underneath they can tell completely different stories.
The most exciting discoveries are often the watches that do not fit the chart.
In vintage G-Shock collecting, the exceptions are sometimes the most valuable part of the story.
The next time you examine a DW-5600C, do not stop at the front of the watch.
Turn it over.
The history may be written on the back.