There are multiple custom mouthpiece makers out there who sell great pieces, and often times, many of them will take requests from customers to modify (slightly) the piece the customer wants to buy. As an example, someone contacts Fred Lamberson and says "I want a J6, but could you add a little more resistance to the curve than you normally would?"
When these pieces inevitably hit the open market, said customer lists it for sale here and "forgets" to put that little part in the ad. Please, if you have one such piece and are going to try to sell it here, put this in your ad so the buyer knows what to expect up front. If every review of a J6 says it's free-blowing and you get one that requires you to drop a reed strength in order to play it, that's not fair to the buyer. And I know this has happened here, more than once.
*the J6 was just an example, and not something that has actually happened (that I'm aware of anyway)