For those interested, page 43 of Schwing's Winchester M21 book gives a reasonably detailed decription of how Winchester did the "machine blueing" process on frames & small parts prior to adopting the Du-Lite caustic salts hot dip process.
Note that Schwing says that machine blueing is also known as charcoal or carbonum blueing. I don't know if he is correct on that point or not.
The results the English get on charcoal blued/blacked triggerguards & furniture are a little different & I suppose everyone had a slightly different process for the results they wanted to achieve & obviously the level of polish done prior to blueing has a significant impact on the results of any blueing process.
