Browning made choke tubes.
Modified describes a choke. Invector is a system of choke tubes.
Invector is a brand not a choke. Invector is a brand not a choke.
I don't think there is an invector plus model. Invector Plus is the choke system Browning uses on newer shotguns.
The Invector, or "Standard Invector" system" was introduced in 1983. A few years later, the Invector-Plus chokes were introduced to fit the new back-bored shotgun barrels. These two choke systems will not interchange. If you have screw-in chokes in your shotgun barrel, the kind of system you have will be stamped on the barrel near the receiver. You must use the proper choke system for your shotgun.
Go to Browning.com and type "Invector Choke" in the search function.
Browning did not make an Invector model. That is the type choke system of the shotgun.
The Invector Plus choke tubes were first introduced in the 1989 Browning Catalog which was published in October 1988. Keep in mind that the Invector was already standard on most shotguns and the Invector Plus was slowing being made available to many models. The Invector Plus choke tube system was available first on BT-99 models and then the BPS and high grade doubles.
The Invector Choke system was first introduced in 1983 and only available on Citori, BPS, and BT-99 models. Fixed choke barrels were still available as well. Please note that the Citori Lightning model wasn't introduced until 1988. All Citori Lightning models in 12, 16, and 20ga. were only available with the invector choke system (and still are). 28ga Lightning Citori's were still fixed choke only.
Invector was Brownings original choke tubes that came out in 1983. Invector Plus came along later for the back bored barrel shotguns. They are not interchangeable. Invector Plus= late model shotgun, Invector = earlier design shotgun. Both are still available for purchase.
Either Invector or Invector+ depending on how it left the factory.
Mid 80's
This project should be probable.I would have a competent gunsmith give you a quote for threading the barrel to except the Invector plus choke system.