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 1973: Flat 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