I too own an FN-49 . The Argie Navy in .308 with detachable mag. I have only fired mine once, just to make sure it went bang.
I'm pretty sure Sarco still has mags for those. I think they were about 75 each, but they ain't gonna be available forever.
I don't suppose you'd post a picture of that thing, would you? I'm kind of hoping one of the Argentine navy guys and one of the Luxembourg sniper guys will throw some pics in of those even rarer FN-49 rifles.