Identical DTBO, different name, one works, one fails

Kernel 3.8.13-bone72. I've taken the Audio Cape DTS and compiled it to BB-BONE-AUDI-02-00A0.dtbo, copy to /lib/firmware, echo BB-BONE-AUDI-02 > $SLOTS, and it works. It creates a working, playable sound card.

I compile the same DTS file to BB-BONE-AUDI-06-00A0.dtbo, repeat the steps above, but echo BB-BONE-AUDI-06 > $SLOTS, and it doesn't work. I rename that to -02-00A0.dtbo, and repeat, and it works.

Inside the file is

  part-number = "BB-BONE-AUDI-02";

So, I change that to -06, and try with the -06 filename. Now the files differ, according to diff, and it still won't load properly.

There is no /lib/firmware anywhere in the filesystem or in initrd*.

What else might be happening here?