The 70C card is almost identical to the 70B card. There is however a small difference on the front of the card.

The coloured area on the card which sits behind the figure is slightly larger than on the 70B card. This pushes the border of this area furthur down the card. On the 70B card, this border is flush with the bottom of the figure name, to the right. On the 70c, this is about 5mm lower. The most noticeable effect of this change is that the copyright statement ((c)1983 Lucasfilm Ltd) in the bottom left corner now falls within the border, so it becomes black text on a white background rather than the white on black text as seen on the 77B card.

As with certain other detail differences on cardbacks, there is no clear logical explanation as to why this might have been done, though it may simply have been due to production taking place at a different location, or the cardstock being printed by a different party.

One other thing noticeable about this cardback is that the card is very thin and flexible compared to the 70B.

It seems as though the 70C card may have appeared at a later date than the 70B as the two ewoks, Paploo & Lumat, appear on this cardback. These figures were released later than the rest of the ROTJ series.
The 70C cards seem less common than the 70B variety. It is not known whether all of the first 79 figures appeared on this cardback, because not so many have been seen it is difficult to establish this. There were figures which did appear on both 70B and 70C cards. There also seems to be a greater range of country of manufacture details on the 70C, all the 70B cards I have seen state Hong Kong as the country of Manufacture, but the 70C cards also appear to have been produced in China and Mexico.
Comparison 70B and 70C cards. Note the different colouring of the Lucasfilm text