These weights are also called calibration weights or precision weights.
OIML Class E1 – Used as a primary reference for calibrating other reference standards and weights where the stability of the environment and careful handling are assured. Class E1 weights have no method of adjustment and are not suitable for general laboratory use. The uncertainty is guaranteed to be 1/3 the tolerance.
OIML Class E2 – Can be used as a reference standard in calibrating other weights and is appropriate for calibrating high-precision analytical balances with a readability as low as 0.1 mg to 0.01 mg.
All Weights with E1, E2 class type are listed in the link below
Cylindrical Knob Type Weights
Milligram weights