Hey I'll have to try the jar shake method on the few of these I've found. I've been holding them loosely, face-up, at about 45 degrees and repeatedly rapping the edge on a hard surface until the back comes out enough to catch with a fingernail.
Mine have been foreign coin hybrids, never double-backs.
I recall the first one I found -- had NO IDEA it was a magic coin, but was fascinated by the fact it sounded completely dead when dropped. No ring, no clink, just thud. I convinced myself someone must have gently bent it into more of a dome shape than flat causing the weird sound. I kept it as a curiosity. Probably more than a year later I saw mention of these magicians coins and wondered if that was what I had. Yep!
Happy Hunting