put some sealant around the half moons at the rear.
+1
This area is the most crucial. "Technically" it shouldn't need any sealant, but it is an important area and it should be done here to prevent any potential problems. A tube of black rtv is inexpensive, like 3 dollars and available at any auto parts store. After you get the rtv it will take you two extra minutes to do the job. Why do the job twice because of a small leak that could have easily been prevented.
I am not an m42 expert,thats for sure, but some things are universal and sealing up odd shapes on gaskets is a must do in my book.