Much like Mr Cumberbatch I’m waging war with the criminal underworld. In the summer of last year I had my second KCB saddle stolen in 3 months. I know I’m not unique here - quite a few customers had got in touch about getting replacements for their own and many more have guiltily admitted replacing it with something cheaper following run-ins with what I can only assume is Moriarty’s posse.
As a result, Kennedy City Bicycles purchased now include security bolts as standard. This means the bolt that holds the saddle to the seatpost and the one that fixes the seatpost to the frame - which currently have two standard allen key bolts - will be replaced with security bolts with their own allen key. While at the moment a cunning felon only need carry around a multi-tool to nick any saddle not attached with a lock, from now on then will find their attempts thwarted by a bolt shape they were not expecting.
As a result of all this you don't need a loop lock for your saddle. The D should suffice since the saddle won’t be going anywhere.