Fr5 would be “Frame 5”. The firewall on a Spitfire (or Seafire!).

Ah! That clears up a lot that I’d been confused about. I though “FR5” was some type/mark of Seafire, and I was wondering why Conners was restoring one Seafire in Missouri for his boss and a shop in England was restoring yet another for the same person. Obviously Airframe Assemblies is subcontracting the work on the firewall for the Mk XV that Conners is restoring.