Black "GI Trousers Belt (Box Type)" used in US ARMY for work other than field battles and for uniforms (dresses). Made by VANGUARD, which has been known as a trousers belt supplier since the WWII, it is exactly the same type as the one delivered to the US Military.
Note) Sometimes, depending on the soldier's preference, it may be used for field battles.

Manufacturer: VANGUARD
Belt:
• Color: Black
• Material: Cotton 100%
• Width: 1 1/4 inch (32mm)
• Thickness: 3mm
• Length: 55inch (139cm)
Buckle:
• Type: Box
• Material: Brass
• Finish: Plated in Gold Color
• Marking: "MADE IN USA" & "SOLID BRASS"
Note) "FB" may be marked instead of above.
• Measurements: 4.8cm x 3.6cm (W x H)
End Chip:
• Material: Brass
• Finish: Plated in Gold Color
• Width: 1 1/4 inch

Note) The length regulation of the belt in the US military seems to be that the belt end is about 1 inch (about 2.5 cm) from the 1st belt loop when worn. Please cut it to the length you like and use it. You can easily open it by inserting a flat-blade screwdriver into the gap between the buckle and belt end.