I have seen this tag/label also, it is found on all the Field Type M barometric Dampers.
No label show at wesite...but bolt can be seen middle right of center...http://www.fieldcontrols.com/m_mg1.php
It is simply a 10-32x1/4" machine screw w/ square nut attached to inside of the damper swing plate. Removing it will obviously leave a 3/16" hole and reduce the weight by that much from the swing plate.
What effect the removal has I can't say???