I just sent 3 pics about my nickel puzzle piece that I don't know
what order to complete the piece.
It's hard to tell from the photos but it looks as though the back
plate is too thin to tap, or at least to get an effective number of
Why screw them together? Why not rivet them.
Anyway, I'd drill first, then tap the back plate, then ream the
holes in the other two plates, test the fit of the screws,
disassemble, solder, reassemble.
Though why you think you can't drill them after soldering on the
back connections I don't know. All you need is a little jig, assuming
you're using a drill press.