Look at the bottom of the balustrade. A baserail is a shaped, grooved rail that the spindles locate into. String capping is a flat board fixed over the top of the string, with the spindles cut and fitted onto it.
If you can see a groove running the length of the rail, you have a baserail and you will need matching grooved parts and fillets. If the top face is flat, you have string capping.
