Since the wedge contact angle on A, B, & C industrial V-belts are the same, you could run smaller A/B-series belts in the larger C-series pulleys in a pinch...they'll just ride lower in the pulley.
The pulleys are made from a single round steel disc that is slitted on its outer circumfrence, then splayed open to created the V-groove shape
This method of manufacture grants this type of pulley more strength verses older 2-part spot welded offerings. Ringball Pulley Catalog