Hi Mike! Thanks to the marketing genius of MagLite, we have a confusing (to some) array of model numbers.
Anyway, the ML designation is used in the current line-up of LED MagLites. Not to be confused with XL lights.
Depending on the exact model, it can cover 2C, 3C, 2D & 3D sizes.
The number generally refers to the user UI options and hence, cost.
ML50 - No frills, On-Off
ML100 - 100%-25%-Strobe-SOS-Momentary
ML125 - same as above, but with rechargeable NiMH pack or 3C Alkaline mixed fuel option.
ML300L - 4 selectable Groups (General, Outdoor, Law Enforcement, Tactical) with various arrangements of Eco, Low, Full, Momentary and Strobe (LED MagCharger also running this UI)
ML300LX - same as above, but with 'aggressive knurled design' and 'matte stealth finish' (or shiny corn-cob knurling to the rest of us.
)
Hope this helps.