Backcheck position jig, determine where to install backchecks. Mark one sample at treble and one at bass.
Connect location from bass sample to treble sample, mark position with a knife, then punch holes. With the holes punched, drill bits will be easier to go in.
Drill and install backchecks at 21 degrees.
Big Bridgeport made in the 1960s in Connecticut. All backchecks are installed.