I believe that the primer system on the 550 and the 750 is the same as the SDB. I’ve owned an SDB and a couple 650s and I prefer the 650 priming although the other works quite well. When the little plastic piece on the bottom of the primer tube starts to wear out I always ended up scratching my head for 10 min trying to figure out why I was suddenly having primer feed issues. It didn’t happen often enough that the diagnosis and fix became automatic like it does with the LNL primer system.
Almost bought a third 650 today, it’s been sitting new and unused since 2018 after loading 500 rounds, but the guy wanted $1,200 for it including a couple toolheads caliber conversions and dies that were all duplicates for me. Too much money and too much trouble. He’d listed it for $850 back in 2018, I’d probably buy it for that today, maybe I’ll make him an offer.