Why is Shimano's 105 groupset called 105?

Question

Shimano's current road lineup is Claris / Sora / Tiagra / 105 / Ultegra / Duraace

105 stands out as odd because its a number. Is there some special meaning ?

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Research

• 105 was released in 1983 as a 6 speed groupset under the name "A105 Golden Arrow"
• At the time Ultegra was known as "600" so some consistency in a 3 digit number. Dura Ace was always called Dura Ace, and Ultegra got its name in 1997
• MTB groupsets have always had a name. There are no numbers for the groupset as a whole, though each year's revision has a model number.

Conjecture from the web

• 105 is the reverse of 501, which came from Levi's Jeans, popular in the late 80s in Japan and matching the "workhorse reliable" nature of the 105.
• 105 reads as "SOI" or "501" when rotated 180 degrees. But SOI is not a Japanese nor English word, like Dura-Ace ( Duralumin alloy + Ace 'best') or Ultegra = ( ultimate" + Integrity)
• In the late 80s Shimano introduced SIS shifting and SLR braking. At this time many groupsets were stopped, like the 200GS, 300GS, 400GS, 500EX, and Exage. New groupsets with SIS/SLR replaced them over time.

So why has "105" persevered ?

Answer 1

This is a non-answer because I've been looking at Japanese bicycling sites and blogs and no one really knows. The closest answer is found here:

105の名の由来とは？

最後に、各グレード名の由来についての豆知識を紹介します。1971年に誕生したデュラエースの名は、素材のジュラルミン（Duralumin）と耐久性を意味するデュラビリティ（Durability）に、世界一のコンポという願いを込めた「エース」を組み合わせた造語です。1976年に誕生のアルテグラは、Ultimate（究極）+Integrate（統合）を意味するコンポーネントです。1982年に誕生した105については、当時のダブルレバーSL-A105の型番を継承し、現在では105というグレード名になっています。

which google translate translate (with some clean-up) as:

What is the origin of the 105 name?

Finally, I will introduce my knowledge about the origin of each grade name [in the Shimano series]. The name of Dura Ace which was born in 1971 is a combination of Duralumin of the material and Durability meaning durability combined with "Ace" which wish of the world's best component. The birth of Ultegra in 1976 is a component that means Ultimate + Integrate. Concerning 105 which was born in 1982, it inherited the model number of the double lever SL-A 105 at that time and now it has become the grade name of 105.

So it's a non-answer answer. The author of this article, Kenji Hashimoto, is a sports journalist in Japan and so it's about as close as one might get unless you ask Shimano.