Standard Invector Choke Tubes. The original Browning removable choke tube system. 
On many shotguns, look for a barrel stamp that says "Invector" or "Standard Invector" Generally this is one of the shortest chokes on the market
They never fit shotguns marked Invector Plus (+) or Invector-DS.

The Standard Invector interchangeable choke tube system offers consistent, dense shot patterns with the right constriction needed for the shooting situation.

Threads on the breech end of the choke tube help prevent gases from slipping between the choke tube and barrel that could damage the choke tube and the barrel.

Also fits most mossberg shotguns such as 590 and 500 series