Sorry guys, I thought this would simply be a get it and install it item and everyone would know what it was. Not important enough to pursue. For the record, all the spring washers I've encountered have been made of spring steel and most had a slight curvature on a cylindrical plane. If indeed they were originally made of spring steel, it's likely they would have rusted away long ago and that would explain the mystery. Anyway, thanks, and I apologize for this one. Les