Hello, I’m a student from Japan.
I should mention that I am writing this with the help of ChatGPT, as English is not my native language.
To start with my conclusion, I am confident that this mechanical pencil is known in Japanese as the “Young Double Knock,” and that its model number is HW-100BK.
I had the opportunity to look through Pilot’s 1977 catalog, where I found a parts price list. That list includes many mechanical pencils that had already been discontinued prior to 1977. When I noticed this particular model number, I saw that the rightmost column specifies the barrel material, which made it relatively easy to infer which model it referred to.
The upper entry reads:
「精アルミ黒染め、ダブルノック式」,
which corresponds to “Precision aluminum, black-finished, double-knock mechanism.”
which corresponds to “Precision aluminum with a hairline finish, double-knock mechanism.”
The lower entry reads:
「精アルミヘアーライン仕上げ、ダブルノック式」,
Additionally, the HW-100SK double-knock is most likely referring to this same model as well.
I hope this information is helpful, and I appreciate all the insights shared in this discussion.