Generally a barrelled receiver is no longer considered in "original configuration" regardless of modification and so loses its C&R status. It's also argued that it has lost it's collectable value.
Whether or not this has been clearly defined, most dealers choose to avoid the issue and require an 01 for any receivers or barreled receivers. Centerfire may have had an issue in the past when it was selling these, or just saw what other distributors policies were and then decided to avoid the issue.
Century has been selling Yugo 59/66 barreled receivers for quite some time now, but never as C&R