I too have seen contemporary adverts from BSA showing the bayonet ends curved slightly downwards throughout the length of the flat end, however judging from the number of completely straight ones around, I think it is possible that BSA made some in both profiles at the factory. Maybe the curved down bayonet could be ordered by the more discerning clients - Interesting to speculate.

As for the bent up ones, well I think they have either taken a whack or two, or the bending is a result of accidentally pulling the trigger with the under lever open. If you think about it, the uncontrolled smashing down of the underlever up to the barrel would create terrific forces which would act progressively along the whole length of the under lever , but would act upon the weakest point of the lever the most. That weakest point would be at the bend of the bayonet end.