Mishra's Factory - Rarity and Peasant/Pauper Legality
C1 means it is an Uncommon even though it was printed on the Common sheet.

Back when Antiquities was printed they only had 2 printing sheets, an Uncommon sheet and a Common sheet, then on each sheet there was a variable number of each card printed to create its rarity. This designation of rarity is written like U1 or C4, denoting one copy on the Uncommon sheet and 4 copies on the Common sheet respectively. U1 cards are considered Rares, U3 and C1 cards have the same rarity and are considered Uncommons, C4 cards are Commons, U2 and C2 cards are a little weird and seem to get classified as Uncommons even though they are actually more rare/common respectively. (Source)

When in doubt you can always check Gatherer. As you can see it considers 3 of the arts to be Rare (since they were printed at U1) and one of them to be Uncommon (because it was printed at C1)