Modified describes a choke. Invector is a system of choke tubes.
Browning made choke tubes.
Starting in 1983.
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.
Best bet is to have it examined by a good gunsmith.
Look at the choke tubes.
Try Mossberg, Weatherby and Winchester
What gauge and grade is it?
Browning Invector.
A 12 gauge shotgun with changeable choke tubes that fires 3.5" shells.
Mid 80's
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.