No filler piece needed
I too think the Mk. 2 is superior. I do like the new mag release tho. One does not have to buy a filler piece for the Mk. 3 to eliminate the LCI.
Field strip the pistol. Look from the bottom of the receiver and you will see a small hole with a pin in it. Tap the receiver a couple of times to allow the pin to come down. Remove the pin. Remove the LCI. You will find a small tab that projects in to the chamber that operates the LCI. Remove that little tab. Reassemble.
You have now disabled the LCI. No gaping hole in side of receiver. I could not get through a full mag until I got rid of the LCI.
Now it works more like my old Mk. 2 I still need to get rid of the mag disconnect.
Freedom is not free