I have no idea....
I've never paid much attention to that as I'm a K98kcollector ..
(Click PIC to Enlarge)
Here's pics below of the same area from her 1916 ShtLE (Short Lee-Enfield) No.1 MkIII* Sniper Rifle (click here) which although manufactured by Enfield and not BSA, looks oblong shape to me too, so maybe they're supposed to be that way?
(Click PIC to Enlarge)
Regards,
Badger