Dan, Well done! Those 2.5mm gaps are caused by the heads of 9.5mm dowel rods which have been inserted into both sides of the inserts and are standing slightly proud. This is obviously to reinforce them. I do not know how long they are, but I expect that they extend well into the base. Similar dowels have also been used to reinforce the butt lands of the inserts at the other end of the chest. Only in this case the maker did a better job and they are flush with the join surfaces.