My 550 is 35+ years old, and has been rebuilt 3 times. It has slowly been upgraded, and Dillon sent me the mechanical, bellcrank powder measure several years ago. It replaced the double springs powering the powder bar.
It didn't run very smoothly; my powder bars are hodge podge going back years, and I leave them set for a particular charge. Some were really binding in the powder measure.
Finally groused to Dillon, and they sent me bunch of current-spec powder bars with the slippery silver coating.
All the difference in the world; bars ran smoothly, and did that little 'bump' at the end of the stroke. I'm guessing that 'bump' is like what we used to do with the tapper on old Lyman powder measures.
In any case, measuring accuracy has improved immensely, and the press runs so much more smoothly.
Don't suffer in silence; talk to Dillon.
Moon