一箇月 is an outdated version of 一ヶ月
The ヶ in this case is not the katakana ケ but a shorthand for the kanji: 箇 and therefore does not produce the "ke" sound.
I got this idea because ヶ kind of looks like the bamboo radical in the top left. Is my assumption correct? Are there other characters that have the same function as ヶ? What are these shorthands called?