Depends on the model.
The M2/M3/M4/M5/M4-2, some M4-P cameras, the various black paint M6/M6ttl editions and the MP, M7 are made from brass. The brass is plated with a very tough layer of nickel (sometimes used to line the interior of gun barrels). On top of that there is a layer of silver or black chrome. You can have these cameras stripped down to the brass, by disassembling them and sending them to a shop like
www.acmeplating.com. It's a fairly toxic process that you don't want to try at home.
The M6/M6ttl and various special editions, with the exception of the black paint models, and some examples of the M4-P are made from a zinc alloy. You do NOT want to strip these, as exposing the zinc to oxygen will case it to oxidize and disintegrate. The zinc is protected by a layer of nickel and on top of that is the silver or black chrome finish.
The situation is similar with the R cameras. The Leicaflex, SL, SL2 and maybe the R3 are made from brass and coated with nickel and chrome.
I believe the R4,R5,R6,R7,R8,R9 are made from zinc alloy (nickel and chrome plated).