Don't know how "old" you mean . . . but I had a 1977 (first year of Miroku, Japan mfg.) Auto-5 Light Twelve. My uncle had purchased it new, and duck hunted exclusively with it. It was a modified choke, which throws a full-choke pattern when using steel shot. After I acquired it, I ordered a new Invector Plus barrel, as I don't duck hunt. I wanted to have interchangable chokes, and shoot lead shot. The Invector Plus barrel fit with no problem.
I have seen them advertised and nI believe they were Hastings. I would check Brownells first. I own a 20 ga lightweight Miroku Japan made and a Sweet Sixteen, Belgium made. They are great guns, older and newer. I don't use steel shot so they are fine with full choke.