That's the one Vince. The square drive is in the end! You could put some leverage on that.......... And if a bolt was really rusted in, as in No5 rifles, then you could almost stop the earth from rotating unscrewing it!
The stock bolt screwdriver was also used to unscrew the internally hidden NUT, buffer on the Bren Guns. This was one of the few spec change in the original Bren drawings, to incorporate oversize slots this to allow the use of this tool