Looks like someone did a nice job of filling and finishing the holes. I have just disassembled mine for painting, and I can assure you that there are holes on both sides.
I would ask a body and paint guy for advice. Of course, his first words will be, "be careful". When you start to drill these, keep in mind that you will be drilling into whatever filler material the last body guy used. I might try a smaller drill bit first and see how that goes before using the correct diameter.
Ray
'68 280SL 4-spd Coupe
