Covex and concave washers

[Brento]

Thanks everyone.

 

[Dave Smith]

Those are spherical washers - used to join uneven surfaces. https://www.mcmaster.com/spherical-washers/

yes, and also used with support jacks on mill setups. if I can find mine I will submit pictures.
Dave

 

[Illinoyance]

Often used with strap clamps that aren't sitting level. There are also spherical nuts to use with concave spherical washers.