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Hello Dan,
Thanks for posting this mystery.
Name likely a house brand for chain store or importer/distributor
There were two of these, the same colour as your example, on Los Angeles CL for
years. Checked just now after seeing your post and they both seem to be gone.
Date has to be right near 1970. As I recall it, ~1971 was the first year for these "safety" brake levers.
Chainset looks to be Takagi - a manufacturer absorbed by Shimano at one point but name kept going. Trade publication advert of 1971.
View attachment 974381
Brakeset appears it may be Yoshikawa. Their logo is a little fish turning in a tight arc. You may find it shallowly stamped in the centre of the front caliper arm of the calipers. Within the area of the company's bicycle products they are best known for their BMX brakes.
The 5V gear block will likely be a Shimano.
Hub barrels may be stamped .333. [Shimano].
Is handlebar stem marked Jun?
The frame's bulge-formed head is by Nikko-Sangyo.
http://www.nikko-bulgeform.co.jp/e/bulge/bulge.html
The machine's fork is an "off the shelf" item not made by the cycle manufacturer themselves. It likely comes from a provider such as Akisu, Ishiwata or Tange. You may find a marking from one of these on its steerer. Me guess would be Akisu.
Probably the single best clue as to an identification of the manufacturer for the cycle is the serial placement. This position on the left side of the head is fairly unusual. It would be a promising avenue for you to explore if you wish to proceed with an investigation.
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