Shield Arms is known for making a number of magazine extensions but it looks like they are also trying to bring a new magazine to market. The S15 is a metal magazine with a 15 round capacity and that fits flush in Glock 43Xs and 48s. That’s right… 15 rounds, flush-fit.
Glock magazines do not usually make very good use of space due to their polymer over metal magazine body construction. The 43X and 48, in particular, have a lot of unused space inside the magazine well. It has been shown online that steel magazines with a 15 round capacity like those from a CZ Compact do in fact fit in the 43X/48 magazine well so the concept of a flush fit, 15 round magazine is not far fetched at all.
The Shield Arms currently shows these magazines at out of stock (not released yet) but they list pricing at $39.95. They have stated that they are working toward a summer release for the S15 mags. If they can pull this off, it may turn into one of those cases where the popularity of an accessory drives the sales of the gun instead of vice versa.