I forgot where I heard this, but I was told that it was considered
good luck for a piano manufacturer to slip a coin under one of the
blocks at the end of the key bed.

A few years ago I was working an Autopiano and, sure enough, there
was a 1917 penny under the block -- clearly put there during assembly.
I've seen this since on other non-automatic pianos, as well.

Karl Van Scyoc
Houston, Texas