Production Variants are pieces that were changed during production for some reason - pieces that broke too often in shipping, or attempts to save money on Krystals. Also the factory was known to do creative pain jobs. These changes were never announced. Generally these pieces are very difficult to find.
Rarity: (ballpark guesses)
C = Common, mass produced produced (1000-25000+ pieces)
U = Uncommon, around 500-1000 pieces produced
R = Rare, around 100-500 pieces produced
VR = Very Rare, less than 100 pieces produced