I don't believe that the bicycle is old as you think. Based on the serial number, it should be 1982, 1956 or 1930. The Japanese did like retro bicycles and continued the use of stirrup brakes into 1980s for their domestic markets but I doubt this is 1980s. At the other end, that is a long serial number string for 1930, given Miyata's production volume at the time. The fender mascot is very indicative of a 1950s model and matches the style Miyata used at the time (i.e. an M inside a cog).
Another sanity check would be the type of bright plating. Commercial chrome plating didn't appear until the late 1920s and most USA manufacturers converted from nickel plating to chrome plating in the very late 1920s or early 1930s. While I don't know the date that Miyata converted, the Japanese bicycle industry typically lagged the US industry by several years during the interwar era, so the presence of nickel would have me leaning towards 1930, while chrome would have me leaning towards 1956.
Miyata didn't have official distribution in the USA until the mid-1970s. Prior to tha,t any Miyata would have been brought in privately. There is much more probability of this having happened in 1956, given the significant military presence that the USA maintained in Japan after the war and though the 1950s.
The missing guard would almost certainly have been a fully enclosed unit. Given the assumed era, the head badge would have been one of their Mister brand versions.
It would be nice to get some additional pictures, especially one from the drive side. Detail pictures of the major components, including any stampings may aid in corroborating the assessment.