> I am going to make a metal xylophone / orchestra bells for an orchestrion
> and I have the sizes from the Orchestrion builders manual, but is there a
> certain type of steel that should be used or is mild steel okay?

Cold Rolled Steel is the industry standard for orchestrions pre-1930.  They
should be tuned based on A = 435; tune your piano and pipes to them.
Be careful when you nickel plate; will lower pitch by at least a few cents.
Best to use a Conn Strobotuner to determine each bar.

Cheers,

S.K. Goodman