L is for Land-Rover, the number is the engineering (and accounting) project number. Yes, there are many 'missing' ones, they are actually there but used for things other than production vehicles, so you'll rarely see them used outside of JLR.
It’s always an issue when every model is badged the same, Range Rover, Mondeo, Avensis etc etc etc at least all the VW Beetles looked exactly the same
Tricky