Not a Seal, Weights & Measure Verification Mark (Edward VII), Image & Found by Tony Hewitt.
Crown / ER / 155
Identified by Paul Cannon, "I am fairly sure that this is part of a lead, weights and measures verification mark. It has been used on a plug of lead inserted into the back of an iron or brass weight. There was a hole on the back of the weight. When the weight was tested and any necessary adjustments made, by the adding of an amount of lead, the verification mark was stamped into the lead plug to indicate the weight was correct.
Beneath the Royal crown are the initials ‘ER’ and what seems to be the numerals ‘155’, which was used in the County of Durham. I believe ER represents Edward VII and not Elizabeth II, whose mark was ‘EIIR’, so this mark dates to 1901-1911. See J.T.Graham, “Weights and Measures”, (1987, second edition), pp 29-30. The following includes an iron weight with the verification mark still in place to compare Etsy - Iron 1lb Weight - Avery, Birmingham."