You will have to call Browning to find out.
Browning made choke tubes.
Go to Browning.com and type "Invector Choke" in the search function.
I don't think there is an invector plus model. Invector Plus is the choke system Browning uses on newer shotguns.
Either Invector or Invector+ depending on how it left the factory.
To change the choke of the shot
Browning did not make an Invector model. That is the type choke system of the shotgun.
Starting in 1983.
Best bet is to have it examined by a good gunsmith.
Look at the choke tubes.
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.
Yes, all Browning Invector and invector plus barrels can shoot steel shot.
Try Mossberg, Weatherby and Winchester