Fox guns used to go to Heinzelman & Son's in NJ for recoloring/hardening w/the cyanide process.
They did the original cyanide case coloring for Fox way back when. The company's been around since WW1 or so.

They still accepted receivers and parts for cyanide coloring as of a few years back.
Don't know if they still do,, or what the quality is now.
There's more than a few 'original' redone cyanide colored guns out there that were sent to them in the 90's and forward.

http://www.manta.com/c/mmbpb30/fred-heinzelman-sons

I don't think they have their own website,,don't recall that they ever did anyway.