They are chrome, all OEM S and RS mirrors are brushed aluminium, none of them are chrome or even polished Aluminium although some do polish them for some reason.
In fact, they are technically acid-etched aluminium finish, alder S4 type (B6) are thick and substantial with rounded edges, lateest, ie. RS4 type (B8.5) ar a lot thinner but are top quality item with nice sharp well engineered edges etc.
I have both, i'll post some detailed pics if anyone interested?