That gray colored material is a "flash coat" of pure tin on later model engines with replaceable bearings. On a poured bearing like yours, I don't have a clue, unless a different alloy might have been used to recondition the main bearings sometime in the past. Lots of strange stuff can happen to an engine in close to 100 years! Look up the "Rotometals" website for a good description of the various alloys of "Babbit" that are available. Babbit metal is a mix of copper, lead, and tin. More copper in the mix gives a brown-looking color and a harder bearing. Chevy used a VERY hard alloy with a lot of copper in the mix for their spray-oiler engines. When I'm machining a connecting rod for an insert bearing I can always tell when I'm whittling on an OEM Chevy rod- - - - -the bearing material dulls the cutter bits a lot faster than one that's been reconditioned with a softer alloy.
Jerry
