OP, where did you buy the smart card reader and cable from?
my t420 doesn't have it as well but i want to get it...
Sorry for a late response. I bought a smart card reader and the cable from Lenovo reseller. Also at the end I managed to get plastic cover which came in the set of two pieces (one with SC slot and normal one which was originally with the laptop. In service parts information it's not clear that FRU contains both covers). Everything together came about 35 EUR. I believe anyone who has service for Lenovo can get this parts.
Here's the list of FRU's that reseller ordered from Lenovo:
04W1639 - SmartCard Reader FPC cable
04W1638 - SmartCard Reader
04W1637 - HDD cover
Hope that helps you. I was also in pain when I realized such business laptop doesn't comes with smart card reader as this was the only configuration available.