Wait, how significantly does the 4003 tailpiece change the tone of the 4001, as compared with the 4001 tailpiece?
There were two different 4001 tailpieces (split and non split), each was a little different. Can't say what the difference in tone was though.
The new 4003 tailpieces seems to be made form the same material..it is a coppery color under the chrome...
I think the copper is an intermediary step in chrome plating. I'd guess the bridges are made from a zinc alloy?