On the subject of colour for differential casings, are you sure they were finished in red primer?
As a Brit with wide experience of sixties British sportscars of various makes, I know that many mechanical components were often finished in Glyptal. This looks like red oxide primer, but is very much tougher and resistant to chipping, wear etc. My Jag XK engine has the internal surfaces painted in Glyptal, which is still available. My Cobra's Kirkham diff is also painted with it.
https://www.electro-wind.com/glyptal...can-361-g.html