I have seen references to these before, but have never had one in hand to examine.
Some have thought they might have been chromed for diving use, but not enough of them have popped out of the woodwork for that to have to be a viable reason for chroming them. The Navy would not have wasted the effort on just a few knives
Someone else thought that they were for ceremonial use, but a MK2 does not get used like a ceremonial chromed bayonet does, on the end of a color guard's rifle. Some M1913 Patton sabers were chromed for a mounted unit in China before WW2, but saber flourishes with a MK2 would look rather ridiculous.
Another theory proposed was that they were post-WW2 surplus sales marketing gimmick knives, but that would not explain the blade marked versions in Pal Blade sheaths.
I have seen nothing in writing that states "The Navy had xxx MK2s chromed for Y purpose." Until someone finds the proverbial "smoking gun" document, the subject remain one of those mysteries open to rampant speculation.