R1A1 is metric, C1A1 and L1A1 are inch.
Sounds like you have Inch-pattern parts kit on metric spec receiver, Century is usually a "FrankenFAL" rifle, using mixed Inch/Metric parts in construction, doesn't hurt the rifle's function, just confusing and parts replacement can be a hassle.
ALL inch barrels with work on most metric builds(exception being Izzie- receiver threads different pitch) and vise-versa. You "might" need to swap gas block, generally going with gas block from parts kit keeps you from opening a whole can of worms, from sight issues to gas tube length.
On to the barrel, Para barrels are getting expensive, if you want a low buck option, look for nice 21" barrel that has been post-ban converted, threads cut off or one of those horrid brakes perm-attached, usually found at or around $100, then find someone who really knows their lathe.