The photo at left is of the indexing holes found in MGB bonnet hinges. These holes were drilled as the bonnets were removed from bodyshells delivered to the Abingdon MG factory by Pressed Steel Fisher. Since aligning the bonnet on re-installation can be a time consuming process (ask me, I know) indexing holes served to make the job much faster at the end of the assembly line.
The bonnets were taken off to make the installation of engine bay components easier and reduce the risk of damage to the painted surfaces. Once the point on the line was reached that no further engine bay access was required, the bonnets were fitted back on the hinges and punches were slipped into the holes to insure a perfect alignment.
If your MGB has these holes it is likely that your car still has the original hinges and bonnet. Or a previous owner put them there for the same reasons noted above. If your MGB does not have these indexing holes it might be a good thing to do before you have to remove the bonnet at some point (*cough* new clutch *cough*).