I have heard that echoed on the various forums, but I have not seen anything concrete, other than perhaps this:
MR73 The unbreakable revolver BROKEN : Revolvers
I asked someone who is a bit of of an expert on MR73s, and he said that to his knowledge Chapuis uses the same ordnance tool steel as the original Mulhouse version.
This seems to be confirmed here:
Wheelgun Wednesday: Manurhin MR73 Review -The Firearm Blog
And here:
Gun Review: Manurhin MR73 Sport .357 Revolver - The Truth About Guns
I have seen others say that it is not the same steel and that it is regular carbon steel. It's hard to know which one is true, because there is scant information available about the differences. I suspect that the available information for the Mulhouse variant is often repeated as being true for the Chapuis variant. As far as I know, the internal design is the same as well.
I will say that the 5-1/4" Chapuis produced version that I currently have has a much nicer trigger than the 4" Mulhouse police variant that I used to have (probably just an adjustment issue). I don't have enough rounds through my Chapuis version to say whether it lives up to the MR73s reputed durability, but it is definitely the nicest 357 that I own.