The only difference I can see between the 2 mags is that the 9 round mag has an extra hole in the side and the spring seems a little shorter, I guess so the spring can compress more for the extra round and is used in the BT380cc. I use them both in my BT380 and BT380cc without any problems. I am not qualified to recommend or try to modify the mags. Maybe a gunsmith would know.
the 9 round is what came on it from the factory,,the 7 round I added the longer one so that my little finger has some place to rest on while shooting I think I got them on ebay worth getting