Sounds like you have headlamps from an early '28 Hupmobile A series car. If so the explanation is simple. According to the Hupp Parts manual, early A series cars came standard with steel, black head and taillamp buckets. Chrome lamps were available as an option, then became standard at car # A115,001.These can be identified because the parking lights were mounted in the headlamp reflector, there were no cowl lights While this was Hupp's practice, it may also been that of other manufacturers as well..
Sorry, I didn't know you are doing an Auburn. But you should remove the mounts from the buckets so any plating solutions can be flushed away and not caught between the mount and bucket to cause problems later. The mounting base can then be restored and reassembled easily. I use 1/4-20, stainless steel carriage bolts. I turn the heads down on the lathe to the shape of the rivet head, polish them and they look "factory", even against the chrome finish.