Eddie, Walt Snyder the Ithaca expert may have some info for you, because I'm guessing your gun was an Ithaca import. I don't know whether there's a breakdown by model on how many 400 Skeet guns were imported. (There was also a Model 400 Field gun, and I don't think many of either were imported--likely fewer Skeets than Fields. Even the less expensive Model 200 Skeet is fairly scarce.) If I recall correctly, about 60,000 SKB side by sides, total, were imported by Ithaca from the mid-60's to the mid-70's. I'd bet no more than a few hundred 400's, total.
I think the confusion on the "sideplated" part comes because in the late 80's, SKB imported some sxs under their own name, and I believe in that series--easy to tell those guns, not marked Ithaca and they have choke tubes--the 400 was a sideplated gun.
