My guess is they bolted the bumper to something to dip it and this is why the plating is the way it is. If you've been in one of these plating operations (and most of the ones I've seen are hot any time of year and pretty nasty), you know they have a large tank of an acid mix into which they immerse items being plated and then have to haul them out again to rinse them a couple of times (different tanks). Small items may be on a wire which goes through a hole (in the part being plated), but larger items can't be done that way. When my grille bars were plated they ran two wires...one on each end through the holes.
Chrome won't rust but it will oxidize. When that happens, air and water can reach the metal underneath and it will rust rapidly because it was stripped before being plated. A friend who did plating until he passed away always told me to wax any chrome plated part with carnauba or brazil wax at least every month. He claimed that was about the only way to protect from oxidation and he also always said to paint the back side of any bumper, grille, etc well and if possible to take it off and paint it again every 3 to 5 years as paint doesn't adhere to chrome well. He said if you couldn't remove the part to paint it with epoxy (catalyzed) paint and hope for the best. From what I was told something about the chrome plating process was changed around 1975 and it wasn't universally agreed to have improved things.
