Just acquired this stainless No. 1 in 416 Rigby. Noticed there is a small section of the chamber on the bottom which has been relieved. I have never seen this on No. 1 before, has anyone else seen this?

Yes that is for a firing pin relief; not to worry. They all have that. You probably never noticed on blue ones as it is hard to see.

Yes, so the firing pin don't snap off on the edge of the chamber