Heinzelman's in Carlstadt, NJ would still cyanide color harden firearms frames and parts in the 90's. Don't know if they still do.
They would do no disassembly, polishing or reassembly,,but that was OK. You had to have an FFL or they wouldn't accept frames from you at that time.

They were the company that did the work for Fox originally AFAIK. They've been around since just before WW1. Last time I sent a Fox to them was around 96 or 97.
More than a few 'smiths and shops were using them.

It's nasty stuff to deal with and the EPA rules & regs surrounding its use, storage and disposal are understandably tight.
K-Cyanide works at around 1600F+,,so it's a bit warmer than the average bone/charcoal case color set up.