The individual component pieces were fabricated (stamped, holes punched, formed, etc) then plated. Date codes were part of the fabrication process. The springs by their very nature, were made of a tempered steel and not plated as the plating process would reduce the temper. The springs were probably made by another company as a sub-contract. Then everything was assembled including the springs, stamped together (deformed edges) and that was a completed sub-assembly. Other items like the bumpers, clips and retainers could have been installed next at this assembly level and shipped to the next process step or assembled on the line. Lubrication was applied somewhere in those steps as well. Those are types of details that we are seeking. No paint was involved.
Jim