unusual to find a 26" Mtn. frame with lugs, certainly rare to find one built with BCM lugs and Simplex DOs. Given that and the splatter paint and serial number, I'd say it's a European-made (maybe French) frame from a major brand and mid-'80s era. First thing I thought was "Peugeot", but looking at their catalogs this doesn't seem to be one. Unicrown fork might be original, but could be a replacement: are the forkends marked?