What are these big shiny metallic "lumps" on the bottom edge of each nozzle of each core of this Falcon Heavy image from the Teslarati article SpaceX reveals Falcon Heavy Block 5 in first official photo, timelapse. See also SpaceX tweet.
I'm thinking it could be related to damping vibrations or oscillations of the nozzle, preventing it from ringing like a bell?
I've never seen one in SpaceX videos of the single nozzle of the 2nd stage, but of course those are very different and much larger structures.
Based on the placement on the eight outer nozzles such that they fall directly between adjacent nozzles, I thought that they might be "bumpers", but that wouldn't explain the presence on the central engine's nozzle. Of course it might be there just so that the engines are identical, so that there aren't two kinds of first-stage nozzles.