STANDARD INVECTOR, INVECTOR-PLUS AND INVECTOR-DS ARE NOT INTERCHANGEABLE. Make sure you know the system that fits your shotgun before proceeding.  For advice on choosing a choke tube appropriate for your shotgun READ MORE HERE. 

“DS” stands for double-seal, our proprietary differential-banded brass seal. It compresses against the barrel wall to keep the choke tube threads clean and makes installation and removal easier than any other muzzle thread system. Invector-DS is standard on A5 and Citori 725 models.