You ask RMK to document that they required proper documentation of the ivory as part of their regular policies. With that, you need only document that your knife is a genuine RMK.

It's like proving a legal case with circumstantial but Logical evidence.

1). All RMK ivory knives were made with documented pre-ban ivory
2). I have a genuine RMK ivory knife
3). My knife was therefore made with documented pre-ban ivory

If you can prove 1 and 2, than you've proven 3 via logic.

Remember the US F&W won't say what proof is required, only that the seller must be able to demonstrate that the knife in question was made with legal ivory. Once you put up logical evidence, it's up to the F&W dept to prove otherwise.

Based on the response they sent me, they don't want to make it difficult for legitimate collectors to sell their items. They just want something.

Something is better than nothing.