Mosby's Cavalry Regiment, formerly the 43rd Battalion, was organized in December, 1864. The battalion was formed in June, 1863, with five companies, later increased to eight. The unit served behind Federal lines in Northern Virginia and was the most effective command of its kind. The enemy forces were never safe and the area became known as "Mosby's Confederacy." In 1865 the unit was still strong with over 600 effectives, but after General Lee surrendered, Mosby assembled his men at Salem on April 21, and the command disbanded. The field officers were Colonel John S. Mosby, Lieutenant Colonel William H. Chapman, and Major A.E. Richards.
http://www.itd.nps.gov/cwss/regiments.cfm (Looked up Mosby, as there's no entry for the 43rd - don't ask why)
So the 43rd Cavalry Battalion is just Mosby's partisan rangers, expanded to battalion strength and given an official organization of sorts.
There were several other (I'm not sure offhand how many other, but quite a few) "Partisan Ranger" commands.
So...um, I'm missing something here about what you're inquiring about.