I didn't know the ヶ in 「一ヶ月」is a small katakana け while it is pronounced or written as か.
ヶ and ヵ are interchangeable ａｓ long as it is not a proper noun like 関ヶ原
I'm not quite sure if ヶ and ヵ are really interchangeable in this sense unless 関ヶ原 can be alternatively written as 関ヵ原.
Are you saying the ヶ read as 「ガ」here means "'s" in this context? So 袖ヶ浦 means 袖の浦?