Hi,
you should have a 13mm hex headed sump plug, these do have a nasty habit of sometimes rounding off, it could be that someone has replace it with something different to get around this.
I have a VW golf sump plug fitted to my Cossie this has a 19mm hex and is drilled and tapped to accept my oil temp probe. This would be impossible with the normal size plug.

CheeRS, Mike.