Circa 1951-1973 – A letter code (dash) number. The letter code represents the year & the number code represents the quarter.

Circa 1974-1976 – A single number code (dash) A single letter code (note, it reverses the number/letter)

Year/Month

1951

1952

1953

1954

1955

1956

1957

1958

1959

1960

January-March

D-1

E-1

F-1

G-1

H-1

I-1

J-1

K-1

L-1

M-1

April-June

D-2

E-2

F-2

G-2

H-2

I-2

J-2

K-2

L-2

M-2

July-September

D-3

E-3

F-3

G-3

H-3

I-3

J-3

K-3

L-3

M-3

October-December

D-4

E-4

F-4

G-4

H-4

I-4

J-4

K-4

L-4

M-4

Year/Month

1961

1962

1963

1964

1965

1966

1967

1968

1969

1970

January-March

N-1

O-1

P-1

Q-1

R-1

S-1

T-1

U-1

V-1

W-1

April-June

N-2

O-2

P-2

Q-2

R-2

S-2

T-2

U-2

V-2

W-2

July-September

N-3

O-3

P-3

Q-3

R-3

S-3

T-3

U-3

V-3

W-3

October-December

N-4

O-4

P-4

Q-4

R-4

S-4

T-4

U-4

V-4

W-4

Year/Month

1971

1972

1973

1974

1975

1976

January-March

X-1

Y-1

Z-1

1-A

1-B

1-C

April-June

X-2

Y-2

Z-2

2-A

2-B

2-C

July-September

X-3

Y-3

Z-3

3-A

3-B

3-C

October-December

X-4

Y-4

Z-4

4-A

4-B

4-C