Procedure for Backcheck installation
Your new obsolete grand action backchecks are made up to be exactly like the ones you sent from your piano project. They are laminated leaving the bottom portion of the last two layers unglued.
These must be glued to the backcheck wires still attached to the piano key. They must be glued but also stay flat for the full backcheck effect. This requires that you cut some thin wooden blocks to clamp onto both sides of the backchecks until all glue is dry. You may do one or two at a time. Make sure the wires are straight is all.
These were originally glued with hot hide glue or burned shellac. They may also be glued with PVC-E or Tacky glue but this will not be historically acurate. If done correctly, these will be tight on the brass wires. Let them dry overnight. You may wish to replace these wires as brass does crystallize over a century and this weakens the metal. You may use standard brass rod/wire of similar diameter of that originally used. Use an anvil and a hammer to flatten the end where the backcheck goes so that that backcheck will not spin and will glue solidly. Some brands flattened the other end that inserts into the key wood, but most just pressed into the key as is or with a sharpened point.
Make a note of how high above the surface of the key the backchecks ride and duplicate that measurement when you get the new ones installed--either the new wires or gluing to the old wires.