Vintage US Military Tool Box Simonsen Metal TypeII Class A Mechanics Cantilever

I've owned this toolbox since the early 1980s. Prior owner painted it red. It's in great shape with only minor surface rust in addition to chipped paint.
The toolbox, was most likely made in the 1950s, not a single exact year but a fairly tight range. Here's how we can tell:
KEY CLUES ON THE METAL TAG
Contract No. GS-00S-31483
This is a U.S. General Services Administration (GSA) contract number. GSA contracts like this began appearing after World War II, primarily in the early–mid 1950s.
Chicago 51, Ill.
This is a postal zone number, which was used before ZIP codes were introduced in 1963. That places it before 1963, and most commonly 1940s–1950s.
Specification: GGG-B-00590
This federal specification was used for government-issue mechanic toolboxes during the Korean War / Cold War era, again pointing to the 1950s.
Manufacturer: Simonsen Industries, Inc.
Simonsen was a major U.S. supplier of government toolboxes during the 1950s, especially for military and federal motor pools.