The BB only has sockets for the chainstays, sort of a mongrel design, but nice because its insensitive to angles between DT, ST and chainstays.
Filing malleable metals is sort of fun. Everything doesn't come off as chips.
Filing has pushed the edge back, leaving a thin layer of metal still connected to the tube.
A lugged BB is used to scribe a like for the cope. Because this represents the inside of the shell, its actually too tight of a curve.
I don't recall having ever seen a clear pictures of this. The Internet now has at least one clear picture now of SLX rifling. Note, as the seat tube, this only has the rifling at the bottom.