I also thought it might be the engine number but it is 6 digits and no letters. So not the engine number.
The 6 digits on the 2nd plate are what’s on the title. That title was issued in 1990. I also have a copy of the registration from 2001 showing the same 6 digit number.
The consignor did not say why there was a second plate. It could be that when the father passed the original title was missing and so a new title was issued with a new number. I really don’t know. Will ask the consignor at some point ….
Thanks for your suggestions as to what might be going on.