Not only that but the cards are slightly smaller. I had to buy sleeves with colored backs.
If your tuck boxes do not have thumb cuts in them then you have one of the more recent print runs and the expansion cards should match.
I have the I.V edition that has tuck boxes with thumb cuts, poker-chips, and rules dated 2007. The cards from my copy of the game seem to be identical in size to standard playing cards.
Are the cards from the Republic expansion identical to what I have, or are they smaller?
The information about this in the BGG Store seems confusing:
One note on compatability: The expansion cards created with the original artwork are 63 mm x 88 mm. The 2007 printing of GtR I.V had cards that were 64.5 mm x 89.5 mm. If you sleeve your cards or have a later printing (which is the most common edition of I.V), then the promo cards will work fine. If not, you will find that the size of the cards is slightly mismatched.