A little confused on the weight guess. But assuming the measurements are correct, I estimate its 40-45g. Weigh it if you can, might narrow the date but its only the marked versions that have been IDed in the past.
This I assume is a free standing counter shaped piece? (& doesn't show signs of iron loops on the reverse? If so its what SS said)
As the weight I guessed is greater than 1 ounce of the old 'Tower Pound' standard & is still much more than the sometimes used 'Paris Ounce'. I can only assume its an unofficial homemade weight, date unknown.