Wow, wow, wow... I've NEVER seen one of these, nor have I even heard of their existence. I guarantee the handset cracked down the middle because it was glued. That light plastic tended to shrink, and with both ends of the handset locked in place, it had no choice but to separate in the middle.
I'm not sure what I'd do with it. It's so rare, I'd be inclined to leave it as it is, in order to prevent further possible damage. On the other hand, I'd be awfully tempted to just "put it right" again. Hard call. Beautiful phone, though, and I think it's a bargain.