[William Tell]

Billthedrill said: "The model number is arbitrary ..."

While what you say is true of most Glock models, including the one used by Cho, I have read that the first Glock model, the G17, was named after the magazine capacity.

[Billthedrill]

I've heard that too, but I believe it's incorrect.

The designation 17 is derived from the gun's being Gaston Glock's 17th patent, rather than its magazine capacity.

The source is HERE.