I'm well aware of what shifter bosses are supposed to look like, and I know perfectly well what an adaptor for a cable stop looks like. I've put bikes together. I've had both downtube and bar-end shifters. This steel spool-like thing is like nothing I've ever seen before. It isn't rubber, it's steel, and it's quite large. My better half hasn't ever seen anything like it before either, and he's put together far more bikes than I have.
I've emailed both the seller and Soma to try to figure out what it's supposed to be for. Since it's in more than finger-tight, I'd like to know what it is before I take a wrench to it to try to unscrew it from the boss.