I think the 410 had "titania" steel barrels and the 409 "ausonia" steel I have seen 409 stamped on the forend timber but never on a 410
accorcing to Wilson the 409-410 series stopped by about 1964 looks like your gun was made in 1968 so my guess is it is a 425 does it have "High Strength Chromium Molybdenum steel" marked barrels?