I was just sorting my collector pack contents, and found a C card.
And realized that these are incredibly easy to spot - at least while you are opening the packs.
the 4th card down is *never* a common base card. and that's exactly where my Horderland 2 'C' version was located.
it looked like it was just a common base card, but because of its position in the pack, I knew that it couldn't be.
I turned it over, and boom, it had a billboard back.