I don't go along with either theory of the belt or trunk. We don't need much debate to prove the belt/buckle theory out of the water & the trunk holds very little water either. None of these trunk plates have turned up with any kind of incised name, which you would expect for an expensive trunk with a brass plate. The general rule on name plates including the silver versions is that more turn up with monograms than without (blank). However, there is far more evidence that these went on leather on horses. 80% of horse brass/blinkers turn up blank & only a few turn up with writing or initials (but we have evidence of both). The timeline of these, specially as its a crude version, may go back to the late 1700s but most of these are 19th C
Crusader,
While the jury may still be out on the actual function and application for these mystery brass plates, usually found with six attachment prongs or bent hooks, I will venture with a rather emphatic debate point that these are in no way Victorian era in the style we see posted at the top of this thread.
There is actually a similarity noted on the links you have posted of UK finds. Is there any provenance of these shown in use as described, in period photos? Surely with an item dating to the later 19th century, or early 20th century, there will be surviving photos of some sort, showing use as horse related hardware. How about catalog listings, with detailed drawn examples?
All the ones I have personally recovered, as well as those found by associates, and furthermore those posted on forums being found in Eastern America, have all been found entirely in context with Colonial period items, dating no later than about the 1820's or so. The one above was posted as being found on a site with a 1786 Nova Constellatio copper coin.
Additionally, in the areas of the United States, west of the Rocky Mountains (where many of my acquaintances have been actively searching sites for well over 25 years),
Not One Single Example of these brass plates with six hooks has ever been seen or found.

The earliest active settlements of any notable European related population, date from about the 1840's onward in the Western American states. In these post-1840 sites, military & civilian relics, coins dating to the 1820's and earlier, and boxes full of every horse related item imaginable have been found, including rosettes, spurs, stirrups, strap buckles, bits, etc.., (not to mention a truck load of horse shoes, oxen shoes, and mule shoes). If these mystery items were indeed Victorian era (1837-1901), and horse related, surely we would encounter at least one excavated example in our post-1840 Western America sites?
CC Hunter