870P With Iron Sight Gun Works Ghost Rings

About 3 weeks ago, I bought an 870 Police Magnum from a local pawn shop. I feel I got a good deal, even though it dosent have a magazine tube extention, it came with ghost ring sights, and is in great condition. I only paid $275 including BGC! My question is, do any of you have any info on Iron Sight Gun Works ghost ring sights? I'd like to switch the brass front bead with a tritium post but don't know the thread size. Also, are they good sights? They are fully adjustable and feel and look sturdy.