Crowley, on my Union Armera I have to take a thin strand of copper wire (bell wire with the insulation stripped off) and thread it through the sideplate screw hole. The end of the wire is placed between the sear and the bent, raising the sear just enough to slide it over the trigger blades. Once the hurdle has been cleared, you simply pull the wire out of the hole and finish seating the lock. No real force is required, and the soft copper wire is not hard enough to scratch anything. Perhaps a version of this technique would help you with your Arrieta?
Anton
