it's a small spot ground smooth on the front counterweight?
with a center punch looking indentation in it. the diameter of the punch mark indicates the resistance or hardness of the metal when compared to the test scale.
http://www.instron.us/wa/applications/test_types/hardness/brinell.aspx?ref=http://search.yahoo.com/search;_ylt=AgUOPlMLc9eiUGDNsN8nYOimN3wV"Enlightened statesmen will not always be at the helm" ~ James Madison
