Unless things have changed recently, Mec-Gar (Italy) was the OEM Manufacturer for Sig-Sauer.
OEM SIG mags have Sig-Sauer in the center of the side of the mag. and are in relatively large, outlined lettering. They also have the typical Zig-Zag or dovetail weld at the spine of the mag.
At one time, Mec-Gar sold Sig mags directly to the public under the Mec-Gar name. They were marked differently than the OEM mags and had the Mec-Gar name and logo on the side. These mags are difficult to find in the States, as Sig stopped Mec-Gar from selling them under their manufacturing agreement (especially the 10+ rd mags). Mec-Gar also sold 15 rd flush fit mags for the P228 and (I think) 17 rd flush fit mags for the P226, which in my experience functioned flawlessly.
If you can find them, Mec-Gar mags are as good as OEM mags for the Sig.