Ah, now I see what you are saying. The regiments 30 etc were Prussian regiments, nothing to do with the Saxons. The titles you give are those listed in the `Stammliste Aller Regimenter und Corps der Koeniglichen-Preussischen Armee fuer das Jahr 1806`.
Regiment 31 was raised in 1729, 32 in 1743, etc, etc.
The regiments 51 etc were raised as of the years 1773 on; nothing to do with the Saxons.
I have checked Jany (Vol II) on the absorption of the Saxon infantry in 1756 - 7; nowhere is there a mention of `Fuesiliers`, but the regimental hat pompons (Hutbueschel) are mentioned, which indicates that they were musketiers, not fusiliers.