They are pistol and sword armed Parliamentarian cavalry troopers. An early unit, armoured with only buffcoats and tri-bar pot helmets. They act as a small counter-balance to the abundance of armour that is found on the ironsides, lifeguards, and cuirassiers.
They're also inferior to the Royalist 'Cavalier' cavalry. This is backed up by the historical evidence that showed the Parliamentarian cavalry of 42 and 43 as being less well trained and not as effective.
Cheers