As far as I know, this was the only variant cover for the Solution. Of these two, the one on the right is far tougher to find. I’ve only come across a couple although that doesn’t mean that there isn’t a big box of these stuck in someone’s garage.
The one on the left is everywhere. If you’re going to buy it you’re better off waiting for a complete run of Solution books; these are often just a few bucks on eBay. The one on the right will require some digging. I bought mine for a couple of bucks from Joe Koch’s web site, but I’m not sure if he has any more.