Look for a 4 digit stamp on your harp. The first two numbers(01-52) correspond to the week and the last two numbers correspond to the year the piano was built.

3581 = 31st week of 1981

0274 = 2nd week of 1974

If your piano does not have a stamp, there are other ways to determine the date. Simply remove a key and observe its key pedestal. 

1970 - 1971: Curved Marcel (Pratt and Reed Key Cap) 

1972 - late 1973Flat pedestal with Front slope (Pratt and Reed Key Cap)

1974: Flat pedestal  (Fat Wurlitzer style Key Cap)

1975- early 1978:  Flat Pedestals , no red felt

Late 1978 - 1981: Straight pedestal with bump mod + red felt

1982-1983: Black Plastic Key, bump with Felt