I believe that the decoders with purple wrap are all Soundtraxx. The one you have looks like one of theirs, and they are the only decoders wrapped in purple, that I've ever seen. TCS has some wrapped in the light blue shrink wrap like Digitrax, but they also have their name stamped onto the shrink wrap as well. Its either the decoder that was mentioned earlier in the thread, or one very similar.
What system are you using to read the decoders with? I'm not sure of other brands but the only current power station that Digitrax has that won't read back CV's is the DB150, found in the Super Empire Builder. Also if you're trying to read on the main, and not a programming track, you'll get the error as well.