A Glock factory 15 round G17 mag would involve the body bulging at the bottom to single-stack dimensions, so it would still take the regular spring, floorplate and follower, but not allow the bottom two rows to double-stack, while still using the same factory spring and follower. Modification would involve hogging out the restriction at the bottom to fit 17 instead of 15 if you lived in a free state.

This follows the pattern of the 10 round G17 mag, where the body was bulged to single-stack dimensions all the way down, internally, resulting in a 10-rd magazine still using the same factory follower, spring and floorplate as the regular 17rd original.