Barrels for the pre 855000 guns and barrels for post 855000 ARE NOT interchangeable.
LeFusil-King Ferry stocked both systems (not available from the current Ithaca factory) and usually had to do some minor fitting on the pre 855000 guns, they then stamped the SN of the gun on the barrel. If you bought 3 pre 855000 barrels and they fit perfectly and head space was good, you were indeed lucky. I am glad the gun is still in use and going strong.