.577 2 3/4"

I was given a .577 2 3/4 brass case, h/s *O.R.B* EXPRESS 577 or *C.R.B* EXPRESS 577
The O/C forms a perfect circle -almost. If I look at it closely it may be letter C. After all, CRB does exist.
So which one is it?
And how many different British sporting calibers was produced by this company?