The numbers are a recording system to show what EMEI's each vehicle has had done to it. (EMEI= Electrical and Mechanical Engineering Instructions. Required modifications specified by higher ups to improve safety or reliability.)
Here is a link to the EMEI's that we have in the library for the trailers.
http://www.remlr.com/emei-trailers.htmlOnce a particular mod has been completed, they drill out or deface the number on the plate relating to the specific mod listed in the EMEI. Often there will be a whole group of minor mods that will be specified for that particular number. At least with the trailer, it probably only has the single mod plate. The trucks and land rovers have a load of them. Most have been removed on my truck, so I have no way to tell if an important mod has been done except by pulling things to bits to find out.
Greg.