I haven't got the Swallow in front of me, but it's a Ti version, and it is almost as though the nut itself is undoing as the frame moves back and forth while being ridden. The leather has the divots settled into it, but it hasn't become like a hammock. When I did tighten the nut, the saddle felt as though it was at the new, unbroken-in stage again, and I eased it off to the point of almost being loose again.

I don't honestly know. I have a Brooks B17 Ti and it hasn't had any issues with the nut/bolt/frame interfaces. And as I have said previously, the Swallow took some time to break in (like about 4,000km, I think) but now it's nicely comfortable 2000km further on.