Southerner -
The console mounted gauge package (RPO U17) was an option. It was
not mandatory with the Z/28 package. In 1969 you could order a tachometer without the console mounted gauge package. However, when you ordered the console mounted gauge package you got a tachometer. So in your case, someone added the console mounted gauges and/or removed the tach from the dash.
Adding the console mounted gauges is most likely what occured.
Paul