That something I just don't understand about the EPA they rather these carburetors polluting because if the will start settings instead of having a qualified tech to fine tune the carburetor. Anyway the Chinese are selling them to the US public.
I went through the same hassle of just trying to get the 21 tooth spline years ago. I finally found an online member of another forum that knew I was a tech in the business of repairs and he sold me one. Then few months later no one could even get them even if they were a dealer. Things have changed since those days but many local dealers still will sell them to even other non dealer shops as they want all the business for themselves. That why I order mine out China now and even a complete or nearly complete set for the same money as I first paid for one tool.
Did you know that MTD go threatened with EPA fines for selling the tools to their dealers without requiring them to also have the special equipment to insure that the adjusted carb still met emissions standards. Also dealers where also threatened with fines up to $41,500 per tool for selling them, and that they could also fine the person that you sold it to up to $4,150. That was the reason that the manufacturers stops selling the tools to their dealers.
Here is also one that most people don't know. A dealer can be fined for installing an engine on a piece of equipment that is older than the engine that was removed. So you cannot legally swap an engine from an older mower to a newer mower. Also you cannot legally adjust the carb on a piece of equipment that you are not an authorized dealer for. So even though your MTD tool will also work on an Echo doesn't mean you can legally use that tool.
I know of one Toro dealer was raided by the EPA because their distributor had released his spring stocking order too early, and that order contained EPA certification stickers for the following year. So he basically received 2014 EPA stickers in December of 2013. For the most part is hasn't been enforced but the potential is there for some real headaches.