How to Date and Identify IBM Master and Secondary Clocks

IBM may be best known today for computers, but in the early to mid-20th century, the company was a major manufacturer of industrial time systems. From factories to schools, IBM master clocks synchronized operations with precision. These clocks, both mechanical and electrical, remain highly collectible and are often found in institutional buildings, surplus auctions, and private collections.

Determining when an IBM clock was made can help establish its historical value, identify correct replacement parts, and guide your restoration process. In this guide, you’ll learn how to read IBM date codes, recognize design changes across eras, and understand what your clock’s features reveal about its production period.

1. A Brief History of IBM Time Systems

International Business Machines entered the clock market in the early 1900s through its acquisition of the International Time Recording Company (ITR) in 1911. Early IBM clocks were often branded as ITR, IBM, or Simplex depending on the production date and regional market.

From the 1920s to the 1960s, IBM specialized in master clocks that controlled multiple secondary (slave) clocks. These were common in:

• Factories
• Schools
• Train stations
• Government buildings

By the late 1960s, IBM sold its Time Equipment Division to Simplex, ending its role in clock manufacturing.

2. Where to Find the Date Code

Dating an IBM clock starts with locating its factory markings. Depending on the model, look for:

• Stamped serial numbers on the back of the movement or inside the case
• Paper or foil labels with printed manufacturing dates
• Stamped codes on the movement plates or pendulum support
• Engraved model numbers on the case back or side

IBM often included the month and year of manufacture either in a direct date format (e.g., 8-47 for August 1947) or as part of a longer serial number.

3. Reading IBM Date Formats

The most common IBM date code formats are:

• MM-YY – e.g., 5-52 = May 1952
• YY-MM – e.g., 52-5 = May 1952 (reverse format)
• Serial prefix – Some models have a letter prefix that corresponds to a production year range

If you’re unsure, compare your code to verified IBM clock serial lists available in horology reference books or collector forums.

4. Identifying the Era by Style and Features

Even without a date code, you can estimate the production era by examining your clock’s design and construction:

• 1920s–1930s – Wood cases, mechanical pendulum master movements, heavy brass plates
• 1940s–1950s – Transition to electric master movements, oak or mahogany cases, and dials
• 1960s – Simpler modernist styling, more aluminum and steel parts, fluorescent dial paint

The dial logo style also provides clues: earlier clocks often read “International Time Recording Co.” while later models simply show the IBM name.

5. Ready to Restore? We Can Help

Once you’ve dated your IBM clock, you may decide to restore it to working condition. IBM clocks—especially master units—have complex mechanisms that require specialized care.

At Clockworks, we offer:

• Professional cleaning and oiling services for both mechanical and electric IBM clocks
• Replacement parts such as hands, glass, suspension springs, and wiring components
• Specialized cleaning supplies safe for brass plates, steel parts, and vintage wood cases
• Precision tools including movement holders, fine tweezers, and screwdrivers
• Technical guidance to ensure the right approach for your clock’s specific mechanism

We use the same tools in our own restorations, so we know they’re safe for antique and mid-century time systems.

6. Common IBM Clock Types and Their Dating Clues

Knowing the model type can further narrow down the production date:

• IBM Master Clock (Model 35, 36, etc.) – Large wooden wall cabinets, often with seconds pendulum; typically 1930s–1950s
• IBM Secondary (Slave) Clocks – Smaller, simpler cases; designed to receive signals from the master; common in the 1940s–1960s
• IBM Program Clocks – Built to control bells, lights, or shifts; usually mid-century production

Photograph your movement, dial, and case to compare with reference examples online.

7. Why Dating Matters

Accurately dating your IBM clock not only adds to its collectible value but also ensures correct parts sourcing and authentic restoration. Installing the wrong style of hands, repainting the dial incorrectly, or using an inappropriate cleaning method can reduce value and authenticity.

Documenting your findings—serial number, date code, case style, and movement type—will help you preserve its story for future owners.

Final Thoughts

IBM clocks are more than functional machines—they are pieces of industrial history. By learning to read their date codes and recognizing era-specific details, you can better appreciate and preserve these remarkable timekeepers.

When you’re ready to restore, maintain, or simply identify the correct parts for your IBM clock, Clockworks.com is here to help. Contact us at clockworks@clockworks.com for expert tools, services, and support.

Updated on: 02/10/2025