Hi,  piano #15465 was built in 1900.  The weight is approximately
700 pounds on the average upright, though it may seem like a ton.
The plate inside is not copper -- it is cast iron.  Copper would not
be stiff enough to support the 40,000 pounds of tension on the strings.
These tended to be high quality pianos.  I have restored eight of them
in the past 10 years.

Steve Grattan - Lost Chord Clinic