I agree with sdchuck1. That sounds like a Yugoslavian magazine. Is it a little less shiny than the rest of the gun, perhaps even flat black?
Shop around at gun shows and online auctions for a replacement, but a) you'll have a hard time finding one, and b) your original matching one is long gone, so it doesn't really matter so much as long as it is an original SKS magazine.
Yes, SKS magazines from various countries will obviously interchange (mostly). I say mostly because the tolerances on manufacturing these things are apparently fairly loose. Sometimes (although not commonly) replacement parts will not fit correctly, even when they are made in the same country as the rifle. I've purchased quite a few "U-fixem" SKS and have assembled a few from parts from all different countries. If parts don't fit, I can usually swap them around on three different rifles and get them all working.
BTW, the Russian SKS are the most desirable (especially Tulas) and are harder to find these days. Try and keep it as original as possible. If you must put aftermarket parts on it, please keep the original parts so it can be restored. SKS fans everywhere thank you.
Oh yea, and aftermarket SKS mags such as USA and those 20 or 30 round Chinese mags don't work. Been There, Done That. Sold them and glad they are gone.
