Hi,
Mark series machines as supplied by the factory only had a single saddle - the bum pad was listed as an optional extra, so, as supplied, the rear mudguard would presumably have been undrilled. Therefore, the holes in your guard could have been drilled by a previous owner for a different brand of bumpad or even a rear carrier. Also, the trend in the late 50's was to fit the pointed nose type of dualseat as used on BSA singles, and the 2 holes in your guard may have been for the rear mounting of that type of seat.
Regards,
Eddie.