I received parts of an 1850s lens today (for spares). The flange does fit some of my other lenses as I'd hoped it would. However it was quite literally black. It was covered with a hard layer of something which was sufficiently well stuck to it that at first I thought that it might be some sort of weird crinkle finish paint. I started cleaning it and discovered that it would come off with elbow grease and hot soapy water although slowly. Eventually it struck me that the coating was black from silver and what I was removing was a ~150 year old encrustation of silver salts from when the lens was used on a wet plate camera! This now explains the finish on this and other lenses, as, when its cleaned, the brass is left with a dull brown colour and missing most of its lacquer, except in the grooves where the encrustation had sealed it in.
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