MMMD would seem to be on to something. I found this listing for a watch that has a box from "Maestros Relojeros Asociados"
http://item.mobileweb.ebay.com/viewitem?itemId=350786805637&cmd=VIDESC
Based on the Spanish forum post and this eBay listing, I would say that "MRA" would have been the Argentine importer for Zenith, rather than the retailer.
gatorcpa