I finally got my gun cleaned, lubed, and reassembled, with Gorilla Tape on the old mag extension clamp. The tape sets it far enough away from the vent rib that it no longer hits the rib supports. It was only hitting on the right side, maybe because the barrel and forend can rotate slightly before they're clamped down. I moved this second mag extension clamp back a hair from its previous position.
I had a hard time getting the gun apart, which is usually extremely easy. You have to reach into the bottom of the receiver and depress the end of the Feed Latch to take the bolt out and to put it back in. The shell lifter moves out of the way so it's easy to reach. The feed latch is a long flat piece of spring steel with a "hook" on the front, that's staked into the right side of the receiver at the rear. But it moved forward a little bit. I slid it back in place and use one of the sidesaddle 2 piece screws to hold it in place to stake it. This is one of those jobs where you need a special staking tool with a big U-notch to go around the other side of the receiver and back inside so you can pound it straight down.. But I don't have one and used a center punch to stake it. The angle was so extreme, it was barely able to get into the steel of the receiver without sliding away. And the rough texture on the side of center punch chewed up the edge of the receiver on the left side. It looked like it was attacked with a rasp. I didn't even try to hammer the metal down in place before I started filing it down. It looks bad but everything is back in working order. A Sharpie Fine Point permanent marker helps disguise this and other scratches, etc.
The TacStar Mossberg 930 Sidesaddle is mounted to the right side of the stock, just ahead of the butt pad, which only extends 1/8" into the stock. I measured the spacing of the 2 holes meant for the screws that replace the Mossberg trigger group pins, and added 2 more with the same spacing, and 4 with the same spacing on the other side. The spacing of the 1st and last hole from the end on both sides was different, but the top and bottom mirrored each other by 180 degrees of rotation. Then I made 3 more holes down the middle, halfway between the other holes. It ended up being 3 diagonal rows of 3 holes, plus 2 holes in opposite corners. I counter sunk all 11 holes and used 1" drywall screws to fasten the aluminum backing plate to the stock. I had to leave 2 screws out because they were hitting the recoils spring tube inside the stock. I stripped 1 or 2 screws but there are still 7 or 8 holding solidly. I ran out of blue Loctite a few days ago, and used a little bit of Permatex high strength RED threadlocker. I hope I never have to take it off, because I'll need to apply heat, and the shell holder is molded from plastic.
I have 6 spare rounds of buckshot on the left side of the receiver, and 6 Brenneke slugs on the right side of the stock in the pics. It really helps balance the gun out, because the hollow stock makes it even more muzzle heavy than the wood stock. I put the slugs away and but the 00 Buck in the shell holder on the stock and it feels so much better that way, I'll probably always carry extra shells there, even if I take the shell holder off the receiver.