Originally Posted by Richard G
I would guess the washer is somehow a method of calibrating the scale.
Thank you Richard,
Then more than one washer should make part of the set, as calibration deviations are not ponderable .This would imply with other washers having been lost.
... Unless the one and only washer serves to calibrate the scale for two specific weighing purposes; two different ounce weight criteria (troy or avoirdupois) or the material to be weighed having two conditions, like gold before and after purification.
... But of course i am shooting in the dark