They were an HK product. In fact, replacement barrels for the ones used in the CF will come with the HK logo on them.
I had a complete CF kit which was let go as scrap from Saskatchewan. The kits were available from the US (I saw them listed in Shotgun news all the time) for around $400 if I recall. But since the 12X's can't be taken out to be fired anymore, why would you want to spend the money on a kit?
The kits are still in use in the CF. The unit I retired from still has FNC1s on the rack with a red band painted on them, just for the .22 subcal capability.