Apologies for triple-posting, but does anyone know what this is, and what it's for? It was in the accessory tray of my Featherweight's case.
It looks like a piece of slate, but seems a bit too heavy for that. It leaves a clear mark on paper, which made me wonder if it was graphite. It's about an inch and a half by three quarters of an inch.
This is a long shot, but could it possibly be an oilstone, for sharpening blades etc?
I have often wondered about the practicality/effectiveness of perhaps sharpening machine needles to prolong their usefulness, and I keep meaning to dig out my oilstone and give it a try.
Does anyone know if sewing machine needles were ever re-sharpened?