That frame appears to be an 81 Pro from the paint scheme, no triple butting in those early years however should be double butted.

Any K in the serial number?

Serial numbers for Miyata Bicycles Made in Japan Since 1972, according with the first letter on the serial number.
"A 1972 B 1973 C1974 D1975 E1076 F 1977 G 1978 H 1979 I 1980 J 1981 K 1982 L 1983 M 1984 N 1985 O 1986 P 1987 Q 1998 R 1989 S 1990 T 1991 U 1992 V 1993 W 1994 X 1995 Y 1996 Z 1997"