I think there was a lot more ferric (brown tape) in the 70's and 80's. Even some looked almost grey.
From something I found on the web:
Quote:
reddish-brown and/or light-brown = basic ferric tapes
chocolate-brown or dark-brown = enhanced (cobalt doped) ferric tapes and pseudo-chromes (cobalt doped again)
dark blueish-black = true chrome tapes
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