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Closeup images of frame details might permit a reader to identify which of the Japan contract builders produced the bicycle. Names such as Kawamura, Kuwahara, Miki, Miyata, Yamaguchi, etc.
Frame's bulge-formed head looks to be a Nikko Sankyo product.
The bicycle's Shimano Stick Shift is a helpful dating aid as there are differing generations of these.
Likewise, the cycle's Shimano Lark rear mech was produced over a number of years with various versions.
Shimano fittings are marked with a date code which is explained here:
http://www.vintage-trek.com/component_dates.htm#shimano
Machine appears to be somewhere near late 1960's.
Manufacturers of japanese produced bicycles can often be determined by the format and placement of the specific machine's serial number. Serials can indicate year, and sometimes even fortnight, of manufacture as well.
If you could post serial and/or image of it the forum's experts might be able to assist...
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