Fuselage: Forward mid fuselage side structure.

The arm rests each have a hole which must be dimpled, yet is inaccessible with my DRDT-2 or with a yoke-equipped squeezer. 

I found that a male squeezer die, the female side of a pop rivet dimpler set placed on a back rivet plate and a strategically drilled 0.187" hole in a footed bucking bar, managed this hole just fine.  Left is the male die in the bucking bar over with the arm reset inbetween that and the back rivet plate with a female die underneath (the latter isn't visible).  Right is everything ready for a hard whack with a hammer.

And here is the dimple.  Not bad.

The seat back adjustment guides need to have their corners radiused (is that a word?).  Turns out I was spot on using my Scotch Brite wheel.