The No5 trailers went through multiple re-builds and reconditioning phases through their 50yr service lives. Probably the biggest of these was the Tenix rebuilds in the mid 90's. This involved a complete strip down to components, recondition/relace/repaint worn parts and then reassemble. Sometimes the original subassemblies did not return to their original trailer. The trailers don't have VIN numbers, so the ARN plates were set aside and re-attached to the "As-new" trailers that emerged from the process. There are numerous examples of this happening to trailers that were "Tenix-ed", coming back out of the rebuild with different chassis's that they went in with.
In your case, 101-001 and 101-163 went in to the reconditioning together, and 101-001 and 101-163 came out together, and the books were balanced.
BTW nice win on e-bay. You outbid me, after the seller sent me more pics of the 101-001 ID sticker!