Working on a Miller and am puzzled by these marks.
Thought maybe someone Bushed the Block at one time but looking from the inside I don't see anything other than a solid wall.
Don't know the history.  Firing Pin is tight, no wearing of the FP Hole. 🤷🏻‍♂️  David

Maybe fire cutting by faulty primers. I had that happen with some Winchester LRM primers a number of years ago. 

Even the bolt gun guys were experiencing the same thing. Winchester was offering to replace the primers and have the bolts repaired at no charge.

They sent an empty 40 foot semi to pick up two cases of Winchester primers from me. 

I was impressed. Got the replacement primers real quick.

That makes sense and would account for the "Blackening" in the Striker area of the Breach Block.
May send it off to get welded.  Thanks everyone👍

  One double charged case will do that, I know from experience.

John Hutchinson had the same thing happen to his Miller action from using pistol primers in cases designed to use rifle primers.

Someone did precise measurements and found pistol primers actually seat deeper since they are not made as long as rifle primers.

The gas pressure forces them backwards on ignition and the weaker structure of the pistol primers bends outwards enough to allow gases to escape which strike the breech face which pits it.

I inspected John's fired pistol primers and there was very definitive black streaks indicating where the gas had escaped around the edges of the German pistol primers he was using.
The pounding back of the pistol primer becomes more pronounced with increasing wear in the primer pockets from more frequent reuse of the case.

Measurements of the different brands of pistol primers indicated some could be seated deeper more than others so that could explain why some people may encounter this problem using pistol primers and others do not.

There are rifle manufacturers like Shiloh Sharps who have stated to never use pistol primers in cases designed specifically to use a rifle primer.

The solution to a pitted breech face is:

1. continue to use it if it is not too severe but with a rifle primer
2. or tig weld the pitted area and resurface the block face
3. cut out the pitted portion and replace it with a bushing.

John's Miller pitting was so extreme that Ron Smith indicated cases could not properly seal so the breech block face needed to be repaired.

The picture indicates pitting on this breech face is not significant and you should be able to continue using it but hopefully with rifle primers.