The ヶ in e.g. 一ヶ月 is a bit of an odd character - it looks a lot like a small version of the katakana ケ, but is it derived from that katakana originally? Or is it a normal kanji? Or is it something ...
Sometime in the early 20th century, usage of the now-historical kana ゐ and ゑ (and their katakana equivalents) dropped off, being replaced with い and え in modern Japanese. What exactly happened here ...