This is the Porcelain Rocket 52Hz frame bag designed for Road frames, in a medium (now a collector's item!). The seams are welded and it has rolltop closure so is completely waterproof. Bought from Chris (inspired ramblings) here, prefer the Tailfin rack I had so going back to using it.

Original ad here

viewtopic.php?f=9&t=20070

Just after what I paid as never used.
£90 posted

I'll take a punt on it. Seems like it might fit the Black Mountain