The requirement stems from
here. Note the (D) in column 3 adjacent to Multi-engine piston (land) indicating that, in accordance with note (b) above,
"although the licence endorsement (column 4) contains all aeprolanes listed in column 2, the required familiarisation or differences training has still to be completed."
LASORS Section F states
"For MEP Class aeroplanes, differences training with a FI or CRI is always required when converting to another type or variant within the class. However, some common sense must be applied; the amount of training will depend on the similarity of the new MEP type to those already being flown. The training must be sufficient to ensure that the pilot can operate the aeroplane systems and operate the aeroplane safely."
JAR-FCL 1.235(1) states
"The differences training shall be entered in the pilot’s logbook or equivalent document and signed by a CRI/TRI/SFI(A) or
FI(A) as appropriate."
So, if you are flying a MEP type other than the Duchess for which you do not have differences training signed up in your logbook, be prepared for the insurance company to wash its hands of you should anything go wrong.