With regards to the previous two posts, you can elivate your worries about your sensor replacement by simply calling Leica and giving them the serial number to determine if you have the newest non cooroding sensor. If you do, you're good to go and enjoy your M9. Likewise if you have the paperwork of your sensor replacement (often written as circuit board replacement) and look at the date of the work performed, this too is an excellent indicator if you have the new non corrosive sensor. If done in 2017 or later, it should be. If done in 2016, you'd need to ask Leica.
All this has been confirmed by many including Leica. A 3rd way is to go into the cameras service menu and many have determined which sensor they have.
That's why when someone sells an M9, if they have proof that they have the non corrosive sensor, the camera will sell for a premium and the buyer can feel comfortable.