I don't believe cyanide case color hardening was or is all that rare. I think it was a cost effective manufacturing process and not reserved for higher end guns and rifles. Chances are millions of H&R Toppers had their receivers finished that way. I believe it's the current 'normal' receiver finish at Shiloh Sharps and pack hardening is an extra cost option.
I also believe alloys that can through harden, such as 4140, are regularly case colored, because I suspect that's what the market wants. I think many of us have seen tons of case colored Rugers, or imported single action revolvers and lever rifles with the look. I suspect the larger case coloring houses are called on to color 4140 regularly.
