I think I might have narrowed the problem down.

NORTHOF60: Your comments of the 303 Ross scared me, so I spent the rest of the day digging through my reference and reloading literature in regards the 303 Ross. I wanted to make sure I wasn't shooting the wrong ammunition in my rifle. I was able to find the 303 ross and 303 Britishicon are one in the same. "303 Ross" was Sir Charles method of self promotion (or at least that is what the general consensus seems to say). As I said, I had an old Mk IIIB that did the same thing to brass, but that chamber was clearly marked with a large L-C (large cone). I thought it odd that a 1908 MkII that was an apparent commercial purchase would have an LC chamber.

I also stumbled on several threads that brought up this same phenomena of odd-looking brass. A common idea amongst these threads is that it aided in extraction.

This was the most clearly written with pictures.

.303 Ross Question - General Ammunition Discussion - International Ammunition Association Web Forum

Interesting aspect of the Ross Rifles, and I learned a few more things that aren't printed in the Ross Rifle Story.