Thanks, Hot. There was not much theft of builder plates until the 1980's, when it began to be noticeable. I recall almost-new Santa Fe rednose GP60M units missing builder plates, with the stubs of rivets still intact. in 1991. I wonder what enjoyment a person could get from having a stolen builder plate from an in-service locomotive.
When I was a kid, I would ride my bike to the Fullerton depot, where I examined the builders plate of every rednose that stopped at the station. By the time I was 13, I could tell you the month and year of construction of just about any Santa Fe passenger diesel that was in service on the Coast Lines. This fueled my interest in railroading, and, particularly in my home road. I never thought that I would later sit at the throttle of some of those same rednoses.
This pastime was possible, because railroad enthusiasts of the 1950's obviously did not think that they needed to personally possess the railroad's air horns and builder plates like they do today. (Yes, some people steal air horns too, off of locomotives tied up at outlying points. The honest horn blowers buy theirs from railroads or scrappers/rebuilders, but there are probably at least a couple of "hot" ones at any big horn-blow, and some are also mounted upon some highway trucks.)