Hi Danny,
               Years ago it was also called a carving mark, when the official registration number along with the registered tonnage were literally carved into a structural member of the vessel in a manner that they couldn't be tampered with or altered. The 24/100 is the registered tonnage measured in 100ths which relate to volume in cubic feet rather than tons weight. I think that the ratio was something like one imperial ton  was  the equivalent of 100 cubic feet of volume.
I vaguely remember Martin Bandy, who was the Amel UK representative explaining this to me when I bought my SM and a new plate had to be fitted. Typically Amel, Bandy never had a written formal contract, simply a gentleman's agreement to sell their boats.
If the second plate is in a similar format, is it possible that your boat got registered in a different country at some point by a new owner, hence the second plate in the engine room?
Best regards,
Trevor Lusty
Ireland