I found a small metal ball inside my wattmeter - where did it come from and should I be concerned?

The metal ball was used as part of the mechanical reinforcement to help keep the feed on the bottom of the housing in place - you can get an idea of the intended location from the callout below. 

We typically learn of this situation with users of the older Model 43 or other portable wattmeters. There is no cause for alarm since the line section does not allow for intrusion and the terminals for the meter movement provide ample separation to avoid shorting.