The buckle is a from the end of a thick strap. Its form is too "generic" to have a more-specific ID than that. The same form, but a smaller size, was on Korean War era US army backpacks.
Although similar-looking ones date much earlier, I believe yours is from the 20th-Century, because it has sharply formed 90-degree edges and internal angles (as if it was constructed out of boards). Earlier ones were more crudely cast.
The bar on it is "back-set" from the frame to allow one thick strap to be attached to the end of the frame and another thick strap to be fed through the frame, atop the bar.