Introducing a new lineup of slim G-STEEL GSTB400 models with new, improved designs made possible by a thin module and carbon core guard structure. The standard model in this lineup, the GSTB400-1A sports a silver color face and resin band. The GSTB400 is made possible by even further miniaturization of its parts and thinning of its modules without sacrificing any Bluetooth or solar function. Power consumption has also been significantly reduced. In addition, the carbon core guard structure of this model helps to make it the thinnest of the G-STEEL Series (as of Apr 2021) while maintaining a high level of impact resistance. The thickness from the back cover to the bezel top is only 12.9 mm, which is more than one millimeter less than the GSTB100 model. In terms of design, the number of button guards has been reduced as much as possible, enabling the use of large stainless steel buttons that are slim yet easy to operate. The top bezel has been given five different finishes, while the middle bezel has been given four different finishes, which creates an elegant effect and changes the appearance of the texture of the metal depending on the viewing angle.