Engine number suffix

Hi All,
Nothing serious, but the engine number on my S1 is 7L 12806 8D
I’ve never seen a letter after the compression ratio; I know at some stage they changed the system to L, S H for low, standard or high, but 8D? It’s on the plate and the block ( matching nos) The car is a Sovereign so is this peculiar to Daimlers?

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