I wonder if there are supposed to be two more slots on the forward end of the mid fuselage skin.
When you make the lower forward fuselage, there are two large -- for want of a better word -- ribs that rivet to the bottom skin and the firewall; these also hold a engine mount bracket/gusset.
You don't rivet the most aft hole on each, though, because those are common to the mid-fuselage skin too.
There are also two outboard stringers (again, for want of a better word, the RV-12 doesn't use part names, only part numbers) that do the same thing, but there are two slots in the skin to accommodate them. Not so for the inboard pieces.
The flanges on all four of these pieces are attach points to the a bulkhead.