Here's another one that was listed for sale on Milweb.
http://www.milweb.net/webverts/54385/f.jpg
Looking at the T joints they appear to have ribbing on the ends and rivets like you get with steel, not copper. I agree with Chris in that I think copper would be too soft. Consider the weight of the set, accessories and batteries and having that bounce up and down.
Tyres are different so I wonder if these were made by a manufacturer or local REME which could account for differences.