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Does anyone know if the ‘Flying D’ bonnet motif on a Daimler 420 is the same part as for the Daimler 250 please?
It’s listed as BD.30177 in the Daimler Spare Parts Catalogue but appears to have been replaced with an after-market part BD23927.

The answer would appear to be 'no'.
There's at least two versions: From above they look identical but from below the stud holes for the bolts to enter are at different lengths along the base.
The one for the 420 needs stud holes at each end of the casting, whereas on the other version, the stud hole at the rear is cast about an inch or so nearer the front and therefore won't line up with the holes in the 420 bonnet.