Cast Iron test weights, calibrated and traceable to National Standards and Trade Approval stamped. Cast Iron calibrated test weights are high quality cast iron weights that are cast in sand moulds and knocked out once set. They are then shot blasted for a smooth finish and blasting grounded on the parts the blasting does not remove. The weights are then black etched painted by dipping or spraying for a smooth black matt protected finished. Test weights up to 2kg are hexagonal in shape and 5kg -25kg are bar weights with a handle.

Trade Stamped weights are for use on typically older scales that use counter balance weights to weigh on mechanical twin pan scales. They have a crown stamped on the underside of the weight.  M1 calibrated weights are to be used to check scales are calibrated all fall within a stated tolerance. 

You should always use weights that are more accurate than your scale to ensure calibration is correct.  Check the weight tolerances here M1 Weight Tolerances