Souless,
Here is how I understand it.
Series i: 250lb bombs, no rockets, Merlin 21, 23 or 25 engines with flame dampers.
Series ii: 250 or 500lb bombs, later rockets as well, some other detail changes, Merlin 25 engines some with, most without flame dampers.
+18lb boost vs +23lbs boost is just 100 octane vs 150 octane and minor engine mods in the same way the Spit LF.IX and XVI use Merlin 66s at +18lbs boost on 100 octane and +25lbs boost on 150 octane. Version of the Mossie isn't relevant other than having Merlin 25s.