Hi,
The silver circle, as you say, is generally less clearly-defined than its magic counterpart. There are various reasons for this, the theory that I tend to accept as the most reasonable is that the period of aggressive market consolidation in the late 90's/ early 00's resulted in many of the firms who would have been considered in those terms were either wiped out or consumed by mergers/ acquired by other firms and now operate on a different business structure. Another reason for the lack of clarity is that there's some of overlap between what might be considered a silver circle firm vs an international/ global elite firm, IE/ GE being a much newer phrase chucked around by journos than the others.
I would tend to consider the silver circle loosely as follows:
Travers Smith,
Macfarlanes,
BCLP,
Ashurst,
maybe Herbert Smith Freehills (although, since the Herbert Smith merger with Freehills, I would personally class them as GE/ IE rather than SC). I've even heard it said (albeit, I think, more flippantly than anything) that
Slaughter and May's business structure is more akin to an SC firm and that it is their profitability more than anything else which earned them the moniker.
Your International Elite firms might include firms like NRF,
Baker McKenzie, (in my opinion) HSF,
Hogan Lovells, etc. There is also a school of thought which would include American-origin firms here (I have it on good authority that they much prefer to be given this banner than just "American") so that would include firms like Kirkland,
Skadden, Lathams,
Sullivan & Cromwell, Goodwin,
Vinson & Elkins, White & Case,
Gibson Dunn,
Dechert etc etc.
As something of an aside, there is (again, in my opinion and only from what I've gathered myself) an increasing discussion surrounding the relevancy of the 'Magic Circle/ Silver Circle' nomenclature. The MC (and SC) handles were created by legal reporters back in the early 90s, when the legal market in the city was an extremely different place and it was used to identify a group of firms who were traditionally British, but had grown to become highly international and major players on the world legal stage. Indeed, a number of MC firms have actually tried to shirk the title (albeit with minimal success, hence discussions such as these).
I am personally of the school of thought that, with time, the Magic Circle term will lose its relevancy and the Global Elite idea will rise to prominence, with the less profitable firms who currently fall into that group maybe gradually forming a bubble of their own.