Richard
Think you might be right that it is a vertical mount. All the hung boxes I have seen have either had strong angle on the edges to support them or heavy backing strips inside to support the bolts. That box looks like there has been no sagging stress with a nice flat surface. If it had been hung it certainly would have bulged the thin metal around the bolt holes.
The box behind the cab on the Inter 6x6 looks very close but different catch.
Lang
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