Yuki looks a bit older here, doesn't she? It's not a bad look for her.

the eyes....

Kyon is touching himself.

That's awesome. I suppose Koizumi's uniform could be used to place the time period precisely, anyone able to do that? As it is, "Meiji" is the best I can do.

The serafuku Nagato's wearing was introduced in Japan after WWI. The Imperial Army switched to khaki in 1911 and the style of Koizumi's shin guntou dates prior to 1934. Dating the uniform itself is problematical, as Imperial officers were apparently required to procure their own uniforms. My best guess would be sometime in the 1920s.

You're absolutely correct.
I wonder why I didn't post translated commentary from the artist's blog in the first place. Here goes:

SOS-dan in period setting of Taisho-era romantic style. Haruhi is cast as a schoolgirl, Kyon is a student / au pair [for want of a better term] with the Suzumiya household, Nagato goes to a different girl's school from Haruhi, Mikuru is a waitress at a cafe, Koizumi is a young commissioned officer in the military.

I also wanted to draw Asakura Ryouko in a furisode [a kind of kimono] and hakama charging at Kyon with a katana, but due to time constraints....

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大正浪漫風な時代設定のＳＯＳ団。ハルヒは女学生でキョンは涼宮家の書生、長門はハルヒとは別の女学校で、みくるはカフェーの女給、古泉は軍人で青年将校、とか。

振り袖袴で日本刀持ってキョンに襲いかかる朝倉涼子とかも描きたいのですが、何時になる事やら……。

Kyon and Haruhi clothes are homage of Sayonara_Zetsubou_Sensei 's Itoshiki_Nozomu and his stalker, Tsunetsuki_Matoi.