This mag came with my 5946 and it doesn't look like a 59xx series mag. Is it factory or aftermarket? It's not a big issue as I shot 100 rounds today without an issue.
That has the old style logo on it. It should hold 14 rounds rather than 15. The early stainless mags that shipped with the first 5900s were also 14 rd maga.
Though uncommon there was also a factory 20 rd mag that will have those same markings on the floor plate.