Why is Shimano's 105 groupset called 105? The current road lineup is 
Claris / Sora / Tiagra / 105 / Ultegra / Duraace

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





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 ?
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 &quot;Ace&quot; 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.