I believe Bosch L is an electronic system with electric solenoid injectors and a mechanical mass flow measuring unit that provides an electric signal as a mass flow analog, and LH is an enhancement to the basic L design that goes back to the seventies. I have no experience with this system, so at this point I'll have to yield to someone with more knowledge of its detailed operation.

Fighting the emission bogeyman is part of the experience of owning an aging car and degradation of emission control performance is part of the aging process. Unfortunately it's often not at all clear what specifically causes the degradation, so you can end up chasing ghosts.

You've still got significant margin, so I would not be too concerned at this point. Make sure your converters are as hot as possible for your next test and go from there.

Duke