Mec-Gar mags came with my Emissary so I consider them OEM and the 4 I have have run flawlessly in that 1911.
Yes, we have all had this exact conversation, about this exact gun, quite a while ago. I have concluded that Mec Gar 25 some years ago is not the same quality stuff it is today. I suspect they are still the lowest bidder though which is why some manufacturers use them for their OEM mags. Turns out I actually have some Mec Gar mags that run well. They are masquerading as Kimber 7 rd. mags.
Incidentally, they are the only ones who are currently manufacturing a 13 rd. magazine for this gun. They sell through someone/thing called Greg Cote LLC. They are $26/ea and currently out of stock.