Over the years, Heathkit test equipment went through a series of style changes, reflecting both the popular styles of the times and decisions by Heathkit's industrial designers. Sometimes the same equipment was reintroduced under a different model number that only differed in style or cabinet colour. While it varied depending on the instrument, and there were many exceptions, I've noticed a few common style themes. Going from the oldest to newest equipment, ranging from the late 1940s to the early 1990s, I've observed the following styles generally used:
1. Battleship gray cabinet with red lettering. Often used black "chicken head" knobs. Example: TS-3 Sweep Generator.
2. Gray cabinet with white lettering. Gray plastic knobs. Example: SG-8 Signal Generator (photo from http://www.nostalgickitscentral.com).
3. Two-tone light and dark gray cabinet with white lettering. Black plastic knobs. Example: IG-102 Signal Generator.
4. Beige cabinet with gray knobs having black centers. Example: IG-57A Sweep generator.
5. Blue cabinet with white front panel. Black knobs, red for concentric knobs. Example: IO-4205 Oscilloscope.
Early equipment tended to be taller than it was wide and incorporated a handle on top. The later style used wider cabinets with handles (if any) on the sides. See, for example, the IP-20 and IP-27 power supplies below which had almost identical functions but different industrial design.
The above is based on material from my book Classic Heathkit Electronic Test Equipment, available
from lulu.com and amazon.com .