I fully agree that having both types of bracket on one truck is unusual. The "in frame" mounts for the hitch spring appear to be placed over the nuts securing the "below frame" mounts as there is an unusual gap between frame and "in frame" brackets. Also, there don't seem to be many nuts inside the "in frame" brackets securing them in place.
On CMPs, I expect to see below frame mounts on trucks where the in frame interfere with other equipment such as access ladder for the C15A Wire5, distribution taps on the water truck.
Or perhaps someone did this to provide extra/double reinforcement to the holes in the rear of the frame..... I have seen those holes damaged when people tried to use a tow cable in an un-reinforced hole.
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